
Class . GLC_2u3 

Book l3 £& T ■ 

Copyright iN° _ 



CSPXRIGHT DEPOSIT. 







A Practical Application of Physics 



HOUSEHOLD PHYSICS 



I BY 

,-v ' 

C. H. BRECHNER 

ii 

FORMERLY TEACHER OF PHYSICS IN THE 

EAST TECHNICAL HIGH SCHOOL 

CLEVELAND 



ALLYN and BACON 

BOSTON NEW YORK CHICAGO 

ATLANTA SAN FRANCISCO 






COPYRIGHT, 1919 
BY C H. BRECHNER 



24 1919 



Nortoooti $ress 

J. S. Cushing Co. — Berwick & Smith Co, 

Norwood, Mass., U.S.A. 



©CI.A536295 



PREFACE 

Household Physics was written primarily for girls. The 
principles of physics in such a book are of course the same 
as in a text-book for boys or for mixed classes. But in 
Household Physics these principles are applied in such a 
way as to interest girls, by using examples and references 
with which they are thoroughly familiar. 

The work was developed in the classroom. At first the 
author used an outline, filling in with applications and 
drawings in the recitation periods. The following year the 
text was written and mimeograph copies put into the hands 
of the students. After this material had been carefully 
worked over with various classes, it was revised into the 
present book. 

The subject of Heat is taken up first, since it is one which 
has many applications of vital importance to the household. 
Thus the girl becomes interested in physics from the first, 
and looks forward to recitations with pleasure. 

The language of the book has been kept as simple as 
possible throughout. The topics are carefully explained 
and these explanations are illustrated by a wealth of line 
drawings and photographs. The problems are especially 
easy and practical. 

The author wishes to take this opportunity to thank the 
several industrial concerns which supplied many of the 
photographs, and also those teachers and pupils who so 
kindly assisted him in bringing the work to completion. 

C. H. B. 
August, 1919. 



CONTENTS 



Chapter 
I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

IX. 

X. 

XL 

XII. 

XIII. 

XIV. 

XV. 

XVI. 

XVII. 

XVIII. 

XIX. 

XX. 

XXI 

XXII. 

XXIII. 

XXIV 



Heat and Heat Measurement 

Expansion . 

Heat Transference . 

Sources of Heat 

Wave Motion 



Sound 



Basis for Music 

Light 

Reflection and Mirrors 

Refraction and Lenses 

Illumination and Candle Power .... 

Color 

Magnetism 

Electricity 

Magnetic Effect of an Electrical Current 
Heating Effect of an Electric Current 
Motion-Producing Effect of an Electric Current 

Induction 

Chemical Relation of an Electrical Current . 

Batteries 

Mechanics of Solids . 

Machines 

Dynamics ... 

Mechanics of Fluids 



Appendix 
Index 



Page 

1 

29 

41 

60 

67 

74 

85 

91 

96 

103 

127 

132 

146 

153 

163 

173 

184 

200 

212 

217 

227 

233 

248 

259 

285 
297 



HOUSEHOLD PHYSICS 



CHAPTER I 

HEAT AND HEAT MEASUREMENT 

1. Nature of Heat. — Physics is a study of the e very-day 
events of life. It is defined as the science of matter and energy. 




Taking the Temperature of Melting Ice to Determine the Melting 
or Freezing Point. 

Matter is anything which occupies space ; e.g. air, water, 
wood, iron, etc. Energy is ability to do work. 

1 



2 HEAT AND HEAT MEASUREMENT 

The student of domestic science must often wonder why 
some of the remarkable things in cooking, freezing, and melt- 
ing happen as they do. One of the divisions of physics, the 
subject of heat, touches closely many of the things done in 
domestic science work ; and it has a vital relation to the coal 
and gas bills at home. 

Heat is one form of energy, and is of two kinds, molecular 
heat and radiant heat; sometimes called sensible heat and 
insensible heat. Sensible heat can be detected by the senses, 
while insensible heat cannot. 

All substances are composed of molecules, or very small particles of 
matter, which are never at rest but are always vibrating with great 
rapidity. In a hot body they vibrate faster than in a cold body. 
When you heat a flat-iron you make the molecules jump faster. If 
you rub your hands together, they are warmed by the increased vibra- 
tion of the molecules. 

The energy which molecules possess, due to their vibra- 
tion, is called sensible heat or molecular heat. Put your 
hand on anything hot and you will see how easily sensible 
heat can be detected by the sense of touch. 

Heat comes all the way from the sun to the earth. It 
travels through air or clear glass without warming it ; but 
when it strikes any object not transparent it is absorbed 
and warms the object. Heat passing through the air is 
insensible or radiant heat, but when it strikes a non- 
transparent object it is changed to sensible or molecular 
heat. 

If you touch a window pane when the sun is shining 
through, the glass feels cold. If you touch a piece of black 
cloth lying in sunshine, it feels warm. 

Either form of heat may be changed into the other. Sen- 
sible heat in the glowing coals of the fireplace starts out as 



TEMPERATURE 3 

radiant heat, or vibration in ether ; but when it strikes you, 
it is changed back to sensible heat. 

2. Hot and Cold. — Hot and cold are common words used 
to denote how a body feels to the touch. They are only rela- 
tive terms, and are not very definite. The term cold is 




Taking the Temperature of Steam over Boiling Water to 
Determine the Boiling Point. 



negative in meaning, and refers to the absence of heat. Cold 
does not come into your house ; but heat goes out, leaving it 
cold, or without heat. 

3. Temperature. — Since the terms hot and cold do not 
give us a definite means of expressing the heat condition of 
bodies, we use another word, temperature. Temperature is 



4 HEAT AND HEAT MEASUREMENT 

the measurement of the speed of vibration of the molecules of an 
object; that is, it is a means of expressing the hotness of a 
body. 

4. Thermometers. — Since the sense of feeling is inac- 
curate, we must have some definite means of measuring 
temperature ; and for such purpose we use the thermometer. 

Most substances expand when heated, and contract 
when heat is removed. This expansion is used to measure 
temperature. Mercury expands or contracts rapidly and 
evenly, and therefore is commonly used as the expanding 
substance in thermometers. Sometimes alcohol containing 
red dye takes its place. There are several kinds of ther- 
mometers, and we must be familiar with two, — Centi- 
grade and Fahrenheit. 

The best way to understand these is to learn how they 
are made, and how the scales are placed on them. First 
we must have some fixed point, that is, some point which will 
mean the same temperature everywhere in the world. Pure 
water furnishes such a point, as it freezes (changes from liquid 
to solid), or melts (changes from solid to liquid), always at 
the same temperature, under uniform atmospheric condi- 
tions. For another fixed point the boiling temperature of 
pure water, under uniform atmospheric conditions, is taken. 

In making a thermometer, take a glass tube of small 
uniform bore with a bulb at one end. Fill it partly with 
mercury, removing all air; then seal it. To put on the 
Centigrade scale, place the bulb in cracked ice and when 
the mercury stops falling make a scratch on the glass op- 
posite this point and mark it " 0°." 

Next place the bulb in the steam just above water boil- 
ing under normal pressure. When the mercury stops rising, 
mark this point " 100°." Divide the space between into 






CHANGING FROM ONE SCALE TO THE OTHER 5 



r 



100 



100 Degrees 



100 equal divisions, each one representing one degree change 
of temperature. This is the most convenient thermometer 
scale we have, since one of the fixed points is at 0°. How- 
ever, since the Fahrenheit scale is more commonly used in 
this countrv, we must learn 
that also and how to change 
from one to the other. 

On the Fahrenheit scale the 
freezing temperature of water 
is marked " 32°," and the boil- 
ing point "212°." The space 
between is then divided into 
180 equal parts, each called a 
degree. 

5. Relation of the Two 
Scales. — The relation between the two scales is shown in 
Figure 1. A little study of this figure will show you that 
an equal space is divided into 100 Centigrade degrees and 
180 Fahrenheit degrees. This means that the C. degree is 
almost twice as large as the F. degree. 



212 



180 Degrees 



32 



Jo 



Figure 1. — Fixed Points on 
the Centigrade and Fahren- 
heit Scales. 



F 

212 

68 F 
36° 
32 



C 

100 



F 

212 



?=20 C ?=68 F 



?=20 




?=36 
32 



C 

100 

20° C 

20° 




a 5 

Figure 2. — Showing how to Change from 
One Scale to the Other. 



That is, 100° C. = 180°F.,or 

1°C. = Hr F., or FF.; and 

180° F. = 100° C, or 
1° F. = ffl° R, or |° F. 

6. Changing from 
One Scale to the Other. 

— To change from one 
scale to the other al- 
ways make a sketch as 
shown in Figure 2, and 
solve as follows : 



6 HEAT AND HEAT MEASUREMENT 

Problem : Change 68° F. to the corresponding Centigrade reading. 
68° F. - 32° F. = 36° F. above the fixed freezing point, (a) Figure 2. 
36 X f = 20° C. above the freezing point on the Centigrade scale. 
Therefore 20° C. is the corresponding Centigrade reading. 

Problem : Change 20° C. to F. reading. 

20° C. is 20° above freezing point. 

20 X f = 36° F. above freezing point on F. scale, (b) Figure 2. 

But freezing on F. scale is 32° F. 

Therefore 32° F. + 36° F. = 68° F., Fahrenheit reading. 

Problems 

1. Change from Centigrade to Fahrenheit readings : 40° C, — 10° 
C, - 40° C. 

2. Change from Fahrenheit to Centigrade readings : 60° F., 22° F., 
40° F. 

3. A change of temperature of 28° C. equals what change of tem- 
perature on the Fahrenheit scale? 

4. A range of 48° F. equals what range on the Centigrade scale ? 

7. Freezing and Boiling Points. — We have already said 
that the freezing point is the temperature at which a liquid 
changes to a solid. Such substances as iron, lead, gold, 
paraffine, and mercury have a freezing or melting point, 
each differing from the others. As you have already learned, 
in making ices and ice cream, putting salt on the ice lowers 
its melting temperature ; that is, a salt solution has a lower 
freezing point than pure water. 

When you put a kettle of water on the stove to boil, how 
do you know when it is boiling? It is not when the vapor 
begins to come from the kettle, but when it bubbles freely. 

If a thermometer is placed in a pan of cold water over a 
flame, the mercury gradually rises. When bubbles begin 
to come out of the water, the mercury becomes stationary 
and will never rise higher, no matter how long or how rapidly 



FREEZING AND BOILING POINTS 7 

you heat the water, if the bubbles are free to escape. If 
you examine the escaping bubbles at such a time, you will 
find that they form at the bottom of the vessel, where the 
heat is applied, rise to the top, and break. They are not 
bubbles of air, but are bubbles of steam, able to push the 
air and water back and thus get out of the water. These 
steam bubbles have the same kind of molecules as liquid 
water, but the molecules are so far apart that they form a 
gas instead of a liquid. 

The boiling point is that temperature at which the vapor 
tension is equal to the applied pressure. The vapor tension 
is the pressure exerted by the molecules of the vapor trying 
to escape. The applied pressure is the pressure of the sur- 
rounding element. 

Freezing and Boiling Points of Some Common Substances 

Under Normal Atmospheric Pressure 



Substance 



Oxygen . . . 
Ammonia . . 
Ether .... 
Methylic Alcohol 
Distilled Water 
Acetic Acid . . 
Turpentine . . 
Fat, Oil, etc. . 
Mercury . . . 



Freezing Pt. 


Boiling Pt. 


Centigrade 


Centigrade 


-235° 


- 182° 


- 75° 


- 39° 


- 113° 


35° 


- 112° 


66° 


0° 


100° 


- 17° 


117° 


- 27° 


157° 


- 33° 


210° 


- 38.8° 


357° 



Hardly any two substances have the same freezing or 
boiling points and some are used for specific purposes be- 
cause of this. Mercury, for example, is used in the ther- 



8 HEAT AND HEAT MEASUREMENT 

mometer because its freezing point is low and its boiling 
point is high. Ammonia is used in the manufacture of 
artificial ice because its boiling point is low. Doughnuts 
are dropped into hot fat instead of water because fat 
boils at about 400° F. and so can be made hotter than 
water. 

8. Effect of Pressure on Freezing and Boiling Points. — 
When water is placed under a pressure it becomes more 
difficult to freeze; that is, its freezing point is lowered. 
Under normal atmospheric pressure water freezes at 0° C. 
or 32° F., but if it is put under a higher pressure it must 
be cooled to a temperature lower than 0° C. or 32° F. before 
it will freeze. 

An example of this is to be had in pressing a snow-ball. 
A good time for snow-balling is when the snow is damp, 
that is, when it is at the freezing point. The loose snow 
is taken in the hands and pressed. This increased pressure 
lowers the freezing point below the temperature of the 
snow, and part of it melts. Then when the pressure is 
removed the freezing point again goes up to 0° C, and the 
melted snow freezes again, making the ball hard. 

If water were put into a strong vessel and sufficient 
pressure were applied, the water would stay a liquid, even 
in our coldest weather. 

The effect of pressure on the boiling point is just the 
opposite of what it is on the freezing point ; that is, pressure 
raises the boiling point. Instead of boiling at 100° C. or 
212° F., the water must be made hotter when a pressure 
above that of the normal atmosphere is put on it. Water 
in the boiler of a locomotive under a pressure of 200 pounds 
per square inch boils at 380° F. instead of at 212° F. On 
the other hand, water under a pressure less than normal 



EFFECT OF PRESSURE ON THE BOILING POINT 9 




Figure 3. — A 
Pressure Kettle. 



atmospheric pressure boils at a lower temperature than 
212° F. 

9. Application of Effect of Pressure on the Boiling Point. 
— Water in an open kettle boils at a comparatively low tem- 
perature on the top of a high mountain 
because the pressure of the air is much 
less than at the sea level. Sometimes 
this temperature is lower than the cook- 
ing temperature of starch ; and so at high 
elevations it is possible to put potatoes 
into an open kettle and boil the water 
freely, without cooking the potatoes. In 
the mountains this difficulty is sometimes 
overcome by using a pressure kettle (Figure 3), that is, 
a kettle with a lid screwed on, making it air-tight. This 
lid holds the steam in the kettle and increases the pressure, 
thereby raising the boiling point above the cooking tem- 
perature. 

Gelatin is a product which comes from the bones of 
animals. To extract it from the bones a temperature 

higher than 100° C. is necessary. 
To get this higher temperature 
the bones are cooked in a closed 
vessel, under pressure. (Figure 3.) 
In the manufacture of sugar the 
principal thing is to evaporate the 
water from the juice of the sugar 
cane or sugar beet. This is done 
by boiling, but when the syrup 
begins to get thick, it is easily 
burned; so it is put into vacuum pans (Figure 4) which 
are closed, and part of the air and steam is pumped out, 



Suction £Er 




Figure 4. — A Vacuum Pan. 



10 HEAT AND HEAT MEASUREMENT 

making the pressure inside lower than that of the atmos- 
phere. This causes the syrup to boil at a lower temperature, 
and so prevents scorching of the sugar. 

10." Quantity of Heat. — Temperature and quantity of heat 
mean very different things. The water in a tea-kettle may 
be at the same temperature as the water in a lake ; yet the 
lake would have much more heat. Even if the water in 
the tea-kettle were boiling, the lake would have more heat, 
though the water in it might be ice-cold. 

The term quantity of heat does not refer to the tempera- 
ture of the body, but denotes the amount of energy in the 
vibration of its molecules. 

11. Heat Units. — The quantity of heat can be measured, 
but not by our familiar units of pound, gallon, foot, etc. 
Other kinds of units must be used, and these are based 
on the effect produced upon water when heat is applied. 
They are B. T. U. (British Thermal Unit), calory, and great 
calory. 

The B. T. U. is the amount of heat required to raise 
the temperature of 1 pound of water 1° F. The calory is 
the amount of heat required to raise the temperature of 1 
gram of water 1° C. The great calory is 1000 calories. 

In these definitions we see that no certain degree is men- 
tioned. This is because it takes approximately the same 
amount of heat to raise the temperature of a certain amount 
of water any one degree as to raise it any other degree. 

Although the calory and B. T. U. are units of two distinct 
systems, there is a definite relation between them. For 
all practical purposes, 1 B. T. U. equals 250 calories, or 1 
great calory equals 4 B. T. U.'s. 

12. Heat of Fusion. — If a piece of ice is placed in a pan 
on the stove, the ice begins to melt; but the temperature 



HEAT OF FUSION 11 

of the water does not rise. Both the ice and the water 
stay at 0° C. or 32° F. until all the ice is melted. After 
that, the water begins to get warmer. The question is : 
Where did all the heat go while the ice was melting? It 
was used to melt the ice. 

As we have learned, everything that occupies space is 
made up of small particles, called molecules. When the 
water is frozen solid, these molecules are drawn together 
by a force called cohesion ; and this force keeps them in 
place. When the ice melts, the molecules are torn apart, 
and slip past one another, making it possible to your the 
water. To tear these molecules apart requires energy; 
and this energy is the heat which melts the ice. 

In other words, we can say : While the ice is melting, 
the heat supplied is used to tear the molecules apart, chang- 
ing the solid to a liquid. 

Some substances require more energy to tear the mole- 
cules apart than others ; so in order to melt some substances 
more heat is required than to melt others. The heat required 
to change a unit mass of a substance from a solid to a liquid 
is called the heat of fusion of that substance. 

If a pound of ice at 32° F. were put on the stove and 
heated, it would have to take up 144 B. T. U.'s before it 
would be all melted. If a gram of ice were used instead of 
a pound, 80 calories would be required to melt it. 

The heat of fusion of ice is the amount of heat required to 
melt 1 pound of ice ivithout changing its temperature. This 
has been found to be 144 B. T. U.'s. (English system.) 

Or, the heat of fusion of ice is the amount of heat required 
to melt 1 grain of ice without changing its temperature. This 
has been found to be 80 calories. (Metric system.) 

On the other hand, when water freezes, it gives out as 



12 



HEAT AND HEAT MEASUREMENT 



much heat as it takes in when the same weight of ice melts ; 
that is, when 1 pound of water freezes, it gives off 144 
B. T. U.'s; and when 1 gram of water freezes, it gives off 
80 calories. 

13. The Refrigerator. — Every one is familiar with the 
refrigerator. It is a box with special walls so constructed 

that heat cannot 



easily get through 
them. A com- 
partment is made 
to put ice in, and 
at least one other 
compartment is 
made to hold the 
butter, meat, fruit 
or any article one 
wishes to keep 
cold. Later a 
more thorough 
study will be made 
of the construc- 
tion of the refrig- 
erator. All we 
are interested in 
now is that it is 
a box in which to 
place ice to keep articles cool so they will remain fresh. 

The ice, when placed in the refrigerator, begins to melt; 
but, to melt, it must have heat. It takes the heat from 
the other things in the refrigerator; and thus keeps them 
cool. For every pound of ice that melts, 144 B. T. U.'s 
must be used up. 




Figure 5. — A Refrigerator. 



FREEZING ICE CREAM 



13 




Figure 6. — Line Drawing of an 
Ice Cream Freezer. 



Two refrigerators can be tested as follows : Place equal 

weights of ice in the two empty refrigerators. Close the 

doors, and note the time re- 
quired for the ice to melt in 

each. The one in which the 

ice melts first lets in the more 

heat, and hence is not so 

good as the one in which the 

ice lasts longer. 

14. Freezing Ice Cream. — 

The freezer in which ice cream 

and ices are frozen is made up 

of two compartments ; one, a 

can, which fits very loosely 

into the other, a wooden pail. 

(Figure 6.) 
The cream, with its other ingredients, is placed in the 

inner can, which, in turn, is placed in the wooden pail. 

Cracked ice, mixed with salt, is packed firmly around the 

can. Then the can is 
kept turning, so that 
the cream will not freeze 
in lumps. But what 
makes the cream freeze 
at all? When the ice 
begins to melt it takes 
the heat from the cream, 
thus reducing its tem- 
perature. 

But the cream would 
never freeze if salt had 
not been put on the ice. 




Figure 7. — Photograph of an Ice 
Cream Freezer. 



14 HEAT AND HEAT MEASUREMENT 

When pure ice melts, its temperature is 0° C. or 32° F., a 
temperature at which cream will not freeze. But when 
salt is mixed with the ice, the freezing point is lowered 
until the temperature has been reduced several degrees 
below 0° C. or 32° F. This low temperature causes the 
cream to freeze. 

Salt is also used to melt the ice on a sidewalk in the winter 
time. The salt reduces the freezing point of the ice to a 
point below the temperature of the air, and so it melts, 
even though the water is still freezing in the gutter. 

15. Getting Heat from Freezing Water. — Sometimes 
when the weather is likely to be cold enough to freeze the 
vegetables and fruits in the cellar, farmers put tubs of 
water in the cellar to protect them. If water is in the 
cellar, it will begin to freeze just as soon as the temperature 
gets as low as 0° C. or 32° F. The vegetables and fruits 
will not freeze at this temperature, because they contain 
solutions of sugar. As the heat leaks out of the cellar, 
more water freezes, giving up its 144 B. T. U.'s per pound, 
and keeping the temperature up to 0° C. or 32° F. 

This goes on as long as there is any water left unfrozen; 
and so protects the vegetables and fruits. Should all the 
water freeze, then the temperature may fall low enough for 
these things to freeze also ; therefore, large tubs are used. 

16. Effect of Heat of Fusion on Climate. — In regions 
near large bodies of water the climate is affected by the 
high heat of fusion of water. The general effect is to make 
both fall and spring come later. 

At the end of summer, as the weather gets colder and 
colder, the water begins to freeze. As it freezes, it gives off 
144 B. T. U.'s per pound, and thus keeps the temperature 
up to 0° C. or 32° F. ; just as putting water in the cellar 



HEAT OF VAPORIZATION 15 

to keep the vegetables from freezing kept the temperature 
of the cellar up to 0° C. or 32° F. This, then, causes the 
fall to be late. 

Again, at the end of winter, when the weather gets warmer, 
the ice begins to melt. In melting, it takes in 144 B. T. LVs 
for every pound; and so keeps the temperature down to 
0° C. or 32° F. ; just as putting ice in the refrigerator keeps 
the things in it cold. Thus, the spring is also late. 

This fact has much to do with fruit-raising. More fruit 
is destroyed by changeable weather in the spring than by 
anything else. If a few warm days come the last of March 
or the first of April, the buds on the fruit trees start. Then, 
if a cold snap comes, the buds are frozen, and the fruit is 
ruined. Xear a large body of water the melting ice may 
prevent a warm period early in the season, so that the buds 
do not start until there is no danger of frosts. 

Problems 

1. How many B. T. U.'s are required to melt 50 lb. of ice in a re- 
frigerator ? Where does the heat come from ? 

2. When a tub of water, weighing 60 lb., is placed in the cellar, 
and it all freezes, how much heat is given up? Where does the heat 
go? 

3. How many calories are required to melt 25 grams of ice at 0° C. 
and raise its temperature to boiling? 

4. If 100 grams of ice at 0° C. are placed in 400 grams of water at 
30° C, and if, after all the ice is melted, the temperature is 8° C, how 
much heat was given up by each gram of ice in melting? 

17. Heat of Vaporization. — If a pan of water is placed 
on the stove and heated, its temperature gradually rises 
until the water begins to boil. After that, the temperature 
remains constant until all the water is boiled away, just 
as in the preceding experiment the temperature remained 



16 HEAT AND HEAT MEASUREMENT 

constant until all the ice was melted. While the water is 
boiling, the heat supplied goes to change the liquid to a gas. 

We have seen that it takes heat to change ice to a liquid 
and that the heat is used to tear the molecules apart. The 
same thing happens when a liquid is changed to a gas. In 
the form of a liquid, water still has the force of cohesion, 
the force of holding its molecules together, so that the water 
stays in a body and remains in the bottom of a vessel. 

When the liquid changes to a gas or vapor, the molecules, 
being much farther apart, do not attract one another per- 
ceptibly, but fly as far apart as the containing vessel allows 
them to go. The energy needed to tear them apart is the 
heat we supply in boiling the water. 

The amount of heat necessary to change a unit weight of a 
liquid to a gas without changing its temperature is called its 
heat of vaporization. 

The heat of vaporization of ivater is the amount of heat 
necessary to change 1 pound of water to steam without change 
ing its temperature. This has been found to be 966 B. T. 
U.'s per pound. (English system.) 

Or, the heat of vaporization of water is the amount of heat 
necessary to change 1 gram of ivater to steam without chang- 
ing its temperature. This has been found to be 537 calories 
per gram. (Metric system.) 

When water vapor or steam condenses, it gives up the 
same amount of heat as was taken in to vaporize it, that is, 
537 calories per gram, or 966 B. T. U.'s per pound. 

The heat of vaporization has many applications in steam 
heating of houses, effect on climate near a large body of 
water, steam cookers, double boilers, etc. 

18. Steam Heating of Houses. — Due to the great heat 
of vaporization of water, steam is very commonly used for 



STEAM HEATING OF HOUSES 



17 



heating buildings. The steam is sent through radiators in 
the rooms, and the 966 B. T\ U.'s per pound, absorbed when 
the water was changed to steam, is given to the air of the 
room when the steam condenses in the radiators. 




Figure 8 



Steam-Heating System. 



There are several systems of steam-heating. Figure 8 
shows one of them. This is called the one-pipe system. 
The steam is led out of the top of the boiler in the basement 
to the radiators in the different rooms. Here it condenses, 



18 



HEAT AND HEAT MEASUREMENT 



gives off its heat, and the condensed water runs back down 
the same pipe. 

To get the steam into the radiator at the start, the little 
stop-cock at the top of the radiator should be opened in 
order that the air may get out and the steam take its place. 
After the radiator is full of steam the valve can be closed, 
and as fast as the steam condenses new steam will flow up 
and take its place. Some radiators have stop-cocks which 

are open when the radi- 
ators are cold, but close 
automatically when 
heated by the steam. 

19. The Steam Cooker. 
— The steam cooker 
(Figure 9) is a closed 
box with shelves. It is 
partly rilled with water 
and set on the stove, or 
directly attached to a 
stove with a separate 
burner. When the water 
boils, the steam fills the 
space about the food on 
the shelves. This hot 
steam cooks the food, 
without danger of burning. The steam cooker is well 
adapted for cooking puddings, custards, etc. 

20. The Double Boiler. — The double boiler is a com- 
bination of two vessels. (Figure 10.) 

The smaller, containing the food to be cooked, is set in- 
side a larger vessel, partly rilled with water. The food can 
be cooked for a long time and cannot burn as long as there 




Figure 



A Simple Steam Cooker. 



DISTILLATION 



19 




Figure 10. — Line Drawing of a 
Double Boiler. 



is water in the outer vessel. The temperature never rises 
above 100° C. or 212° F. 

21. Distillation.— The ques- 
tion of pure drinking water is 
of vital importance, especially 
in large cities. Sometimes 
chlorine is put into the water 
to kill the germs. As chlorine 
is very distasteful to some 
people, they prefer to buy, or 
prepare, distilled water. 

The process consists of boil- 
ing the water, converting it 
into steam, and then con- 
densing this steam, thus pro- 
curing pure water. Figure 12 shows the principle used 
even in large establishments. 

Water is heated in a boiler (B), and the steam is conducted 
through a pipe to a coil (C), in a tank of running cold water. 

The cold water is supplied 
by a hose from the city 
water main to the point a, 
and when warmed flows 
out of the opening b into 
the sewer or into a tank. 
The steam, passing through 
the coil, is condensed, giving 
up its 966 B. T. U.'s per 
pound to the cold water, 
and then runs out of the coil 
as pure water. It is pure because only the water will evapo- 
rate ; hence only pure water vapor is in the coil to condense. 




Figure 11. — Photograph of an 
Aluminum Double Boiler. 



20 



HEAT AND HEAT MEASUREMENT 



Distillation is used to refine other substances, such as 
alcohol and turpentine. But in these cases the substance 
has to be distilled several times, and the process is then 
called fractional distillation. 

In the case of alcohol, the liquid which contains the 
alcohol is placed in a boiler and heated, the temperature 




Figure 12. — Diagram of a Simple Distillation System. 

being kept at the boiling point of alcohol, which is below 
the boiling point of water. The alcohol vapor is driven off, 
but with it a little water evaporates. When this is con- 
densed again, it still contains some water. This new liquid 
is again distilled, yielding a product more nearly pure alcohol. 
This process is kept up until the liquid is as nearly pure as 
desired. 



ARTIFICIAL ICE PLANT 21 

22. Other Applications of Heat cf Vaporization. — In 

the summer time, regions far inland get very warm. But 
near a large body of water the heat is less intense because, 
in evaporating, the water takes up 966 B. T. U.'s for every 
pound evaporated; and thus keeps the temperature lower 
than it would otherwise be. 

You have probably noticed that the air gets cooler after 
you have sprinkled the street or lawn. The water on the 
ground begins to evaporate, taking heat from the ground 
and air, thus lowering the temperature. The same thing 
occurs after a rain. 

Nature uses the same principle to keep your body cool. 
When you exert yourself strenuously, or when the day is 
warm, perspiration is thrown out to the surface by the skin. 
This perspiration evaporates, taking the heat from the 
body to do it. Would you get as cool if you removed the 
drops with your handkerchief ? 

23. Artificial Ice Plant. — In making artificial ice, the 
same principles apply as in natural evaporation and freezing. 
The ice freezes as naturally as the ice on a lake. The only 
artificial part is the producing of the low temperature. 
Nature does the rest. 

The artificial ice plant (Figure 13) consists of four prin- 
cipal parts: a cooling coil (A) for the ammonia gas; a 
force pump (P) for compressing the ammonia gas; an 
expansion coil (B) where the brine cools; and a freezing 
tank (C) where the ice is frozen. 

The operation of the plant is as follows : the force pump 
P draws the ammonia gas through the valve d and forces 
it through the valve e, under high pressure. From here it 
is led through the coils in the tank (A), where it is cooled 
by running cold water. 



22 



HEAT AND HEAT MEASUREMENT 



As the gas, under high pressure, becomes cool, it con- 
denses and is led out of the coil at the bottom as liquid 
ammonia. At the stop-cock / the liquid is allowed to flow 
through slowly, and there it turns to a gas and expands sud- 
denly. This evaporation and expansion require a great 
amount of heat. 

As this evaporation and expansion take place in the coil 
in the tank (B), the heat is taken from the brine in tank (B), 




Figure 13. — Diagram of a Simple Artificial Ice Plant. 



thus reducing its temperature several degrees below 0° C. 
or 32° F. The ammonia gas then passes on up to the force 
pump, to be again compressed and used over. The cold 
brine is pumped from tank (B) to tank (C). In (C) are 
placed the molds containing pure water. The heat passes 
from the water to the brine, and thus the water freezes. 

In iceless refrigerators cold brine is pumped through 
coils just as in the artificial ice plant. Modern meat mar- 
kets use this method. 

The ice in artificial ice skating rinks is frozen by the method 



WATER VAPOR IN THE AIR 23 

above. Coils of pipe are placed on the bottom of the 
floor, and then enough water is run over it to cover these 
pipes an inch or two. Brine is pumped through the pipes, 
which in turn freezes the water. In this way ice skating 
can be had at any time of the year. 

Problems 

1. Find the heat required to evaporate two pounds of water with- 
out changing its temperature. 

2. Find the heat required to evaporate 1500 grams of water with- 
out changing its temperature. 

3. If, in making jelly, one half of the weight of the juice is boiled 
away, how much heat is required to make one quart of jelly? (Take 
weight of juice as eight pounds per gallon, and starting temperature 
as 62° F.) 

4. ^When ten pounds of steam is condensed in your radiator, how 
much heat is given to the room? 

24. Water Vapor in the Air. — When water is boiled 
away in a tea-kettle or a pan, or when it evaporates from 
any body of water, the water seems to disappear; but it 
does not go out of existence. It simply goes into the air 
and is invisible. The molecules of water vapor mix with 
the molecules of other substances in the air, of which they 
become a part. 

There is a limit to the amount of water vapor that the 
air will hold, and this limit depends upon the temperature 
of the air. The warmer the air, the more vapor it will 
hold. 

When the air contains all the water vapor it will hold, it 
is said to be saturated, or to have reached the saturation 
point. The saturation point depends upon the temperature. 

The following table shows the vapor tension of water 
under normal pressure at different temperatures. 



24 



HEAT AND HEAT MEASUREMENT 






Temperature 


Vapor Tension 
(cm. of mercury) 


Temperature 


Vapor Tension 
(cm. of mercury) 


o°c. 




0.460 


21° C. 


1.862 


16° C. 




1.362 


22° C. 


1.979 


17° C. 




1.440 


23° C. 


2.102 


18° C. 




1.546 


24° C. 


2.232 


19° C. 




1.645 


25° C. 


2.369 


20° C. 




1.751 


100° C. 


76.000 



25. The Hygrometer. — An instrument used to measure 
the amount of water vapor in the air is called a hygrometer. 
Figure 14 shows a common form of the hygrometer. It 
consists of a small spring, a pointer, and a scale. The scale 

denotes the per cent 
of water vapor in the 
air, complete satura- 
tion being 100 per 
cent. For example, a 
reading of 65 per cent 
means that there is 
65 per cent as much 
water vapor in the air 
as it would hold if 
saturated. 

By knowing the 
weight of vapor re- 
quired at a certain 
temperature to satu- 
rate the air, with the hygrometer reading it is easy to com- 
pute the exact weight of vapor that is in the air. 

If saturated air is heated to a higher temperature, it will 
hold more vapor ; but if saturated air is cooled it will hold 
less, and some of the vapor must condense. 




Figure 14. — The Hygrometer. 



SXOW AND HAIL 25 

26. Dew. — If warm air comes in contact with a cold 
object it may be cooled below the saturation point and some 
of its water vapor may condense and appear as drops on 
the cold object. These drops are called dew. You have 
all seen a pitcher of ice water sweat in the summer time. 
The pitcher does not really sweat, but merely has dew on it. 

Dew also forms on grass and on the leaves of trees. Dining 
the night small objects, such as the grass blades and leaves, 
radiate their heat ; and thus become cooler than the surround- 
ing objects. These grass blades and leaves then cool the 
air that touches them, and dew forms when the air is moist. 

27. Fog and Clouds. — If a cool current of air strikes a 
warm current, the warm air is cooled below the saturation 
point, and the surplus water vapor condenses in very small 
particles, but large enough to be visible. If this condensa- 
tion occurs near the surface of the earth, it is called fog. 
If it occurs high in the air, it is called clouds. The greatest 
fog region in the world is just off the banks of Newfound- 
land, where the cold air from the north meets the warm 
air from the Gulf Stream. 

28. Mist and Rain. — If, in the case of fog, the condensed 
particles become sufficiently large to fall slowly, they are 
called mist. If these particles become large enough to fall 
rapidly, they become drops and are called rain. 

29. Snow and Hail. — When the water vapor is forced to 
condense at a temperature below the freezing point, the 
small particles freeze as they condense and form snow- 
flakes. The flakes get larger and larger as they come into 
contact with one another in the air. 

The formation of hail is more complex than that of the 
other forms of condensed water vapor we have noted. 
Scientists are not entirely agreed as to the facts concerning 



26 HEAT AND HEAT MEASUREMENT 

the process. The theory generally accepted is that a small 
particle of water is condensed and frozen high up in the air. 
It starts to fall and collects on its surface a layer of water ; 
but before it hits the earth it is carried up again by an up- 
ward current of air. This water freezes on its surface, 
while at the high altitude, forming a new layer of ice. Again 
it starts to fall, and collects a new layer of water, only to be 
carried up again by another upward current. This process 
is repeated until the hail stone becomes so heavy that it 
cannot be carried up any more. 

This theory of formation is based upon the structure of 
a hailstone. When cut open, it is found to be made up of 
distinct layers ; some of clear ice and some of snow ice. 

30. Heat Capacity. — If you heat a five-pound flat-iron 
to the boiling point, and place it in a pan of cold water, and 
if you then pour five pounds of boiling water into another 
pan containing an equal amount of equally cold water, you 
will find that the five pounds of boiling water have made 
the pan into which it was poured much warmer than the 
flat-iron has made the pan in which it was placed. 

What conclusion would you draw from this? Note that 
the weights of the boiling water and the hot iron were the 
same; that they were at the same temperature; and that 
they were put into the same weights of water, which were 
also at the same temperature. The answer is, the water 
contained more heat than the iron. Different substances 
hold different amounts of heat at the same temperature. 
In other words, they have different capacities for heat. 

The definitions of our heat units are based on the heat 
capacity of water. We say that when 1 gram of water is 
heated 1° C, a calory is put into it ; and that, if 1 pound of 
water is heated 1° F., a B. T. U. is put into it. 



SPECIFIC HEAT 27 

But if a gram of any substance other than water were to 
be heated 1° C, it would not take exactly 1 calory, but a 
certain fraction of a calory, depending upon the substance. 

The heat capacity of a substance is the heat required to 
raise a unit weight of the substance 1°. If it is in the English 
system, it is the number of B. T. U.'s required to raise 1 
pound of the substance 1° F.; if it is in the metric system, 
it is the number of calories required to raise 1 gram of the sub- 
stance 1° C. 

31. Specific Heat. — As the heat capacity of pure water 
is uniform, substances having different heat capacities are 
compared with water as a standard. From this comparison 
we get the term specific heat. The specific heat of a sub- 
stance is the ratio of the heat capacity of the substance to the 
heat capacity of pure water. 

Eliminating the idea of heat capacity, we can define specific 
heat in this way : Specific heat is the ratio between the amount 
of heat necessary to raise a certain weight of the substance 1° 
and the amount of heat necessary to raise the same weight of 
pure water 1° ; or 

./. tt Heat to raise substance 1° 

specific Heat = 77 — — ; ; r-j- — -„ — -3 

Heat to raise equal weight oj water 1 

Table of Specific Heats of Some of Our Most Common Substances 

Substance Specific Heat 

Aluminum .22 

Brass 094 

Copper 095 

Iron 1138 

Mercury 038 

Lead 031 

Ice 5 

Air (at constant pressure) 2375 

Hydrogen (at constant pressure) 3.4 

Steam (at constant pressure) 48 



28 



HEAT AND HEAT MEASUREMENT 



32. Application of Specific Heat. — The 

high specific heat of water has a powerful 
effect on the climate of regions near a large 
body of water. This effect is the same as 
that produced by the high heat of fusion. 
The principle is slightly different, for the heat 
is used to raise the temperature of the water, 
instead of to melt the ice. (See § 16.) The 
effect is much greater than it would be if the 
Hot Water body were mercury or alcohol or any substance 
Bottle. whose specific heat is less than that of water. 

The hot water bottle is an application of specific heat. 

It is better than a hot flat-iron or other hot object, not only 

because it is more convenient, but also because it holds more 

heat. 




CHAPTER II 
EXPANSION 

33. Expansion. — One effect of heat is to make the 
molecules of a body vibrate faster. This increase in speed 
causes the molecules to take up more space. The mole- 
cules themselves do not get any larger, but they require 
more free space in which to vibrate. 

Suppose a number of people were to stand close together, 
with a large rubber band stretched around the whole crowd. 
If all stood perfectly still, they could get into a compara- 
tively small space. But if every one began swaying and 
elbowing his neighbor, each person would take up more 
room, and consequently the space occupied would be larger, 
and the rubber band would have to stretch. 

This is what takes place when a body is heated ; and we 
call it expansion. Expansion is the increase in length or 
volume of a body. 

34. Coefficient of Linear Expansion. — All substances do 
not expand at the same rate. For example, a bar of iron a 
foot long would not expand as much as a bar of brass a 
foot long, if both were heated through the same range of 
temperature. In order to have a way of expressing how 
much a substance expands we use the term coefficient of 
linear expansion. 

The coefficient of linear expansion of a substance is its 
expansion per unit length per degree C. 

29 



-60 cm- 



30 EXPANSION 

Suppose a bar of aluminum, 60 cm. long at 25° C. (Figure 
16), gets .1 cm. longer when heated to 100° C. The in- 
crease in temperature 
25 C to 80 C . OKOr . , im o r 

from 25 C. to 100 C. 

is 75° C. If the bar 
1 cm ex P an ds .1 cm. for 75° 
Figure 16.- Expansion of a Rod. q j t wffl expand ± 

cm. for 1° C. If 60 cm. expand zr cm., then 1 cm. will 

i o 

expand ^„ ' ._ cm. or '. cm. = .000022 + cm. 
75 X 60 4500 

The number .000022 is called the coefficient of linear ex- 
pansion of aluminum. 



Table of Coefficients of Linear Expansion 

Substances Coefficient 

Aluminum 0000222 

Brass 0000187 

Copper . .000017 

Glass 0000083 

Iron 0000112 

Platinum 0000088 

Steel 000013 (tempered) 

Steel 000011 (untempered) 

If the range in temperature is given in F. degrees, then the above 
coefficients must be multiplied by f . 

35. The Thermostat. — The thermostat which regulates 
the heat of our rooms uses the principle of expansion. It 
is constructed as shown in Figure 17. The pointer (P) is 
made of a strip of steel (S) and a strip of brass (B), laid 
side by side and fastened so that they cannot slip on each 
other. One end is fixed, and the other end is free. Electric 



THE THERMOSTAT 



31 





connections are made as shown in the figure. The battery 
(Bat.) is placed in the circuit, together with two magnets 
(Mi and i¥ 2 ). 

The thermostat is placed 
in the room, the temperature 
of which is to be regulated, 
and the magnets (Mi and 
M 2 ) are placed in the base- 
ment. The wires lead from 
the thermostat to the mag- 
nets. When the room gets 



Figure 17. — Diagram of a Thermo- 
stat and System. 

too warm, the two metals ex- 
pand ; but the brass expands 
the faster. This makes the 
pointer bend and touch the 
connection x, thus operating 
magnet M 2 . Magnet M 2 re- 
leases a spring which closes 
the draft of the furnace, and 
this allows the room to cool. 
When it gets cool enough, 
the two metals contract ; but 
the brass one contracts the 
more. This makes the pointer 
bend in the other direction, 
and it touches the contact point y. This operates magnet 
Mi, which releases a spring opening the draft. In this 



Figure 18. — Photograph of the 
Sensitive Part of a Thermostat 



32 



EXPANSION 



way a room may be automatically kept at an even 
temperature. 

36. Compensating Pendulum of a Clock. — The pendulum 
of a clock is the regulator which makes the clock run evenly. 
If the pendulum is too short, the clock runs too fast; and 
if it is too long, it runs too slowly. 

Since metals expand when heated, a clock 
will not run correctly at different tempera- 
tures unless a special pendulum is arranged. 
When a pendulum is so arranged that a 
change in temperature does not affect it, it 
is called a compensating pendulum. 

One kind of compensating pendulum is 
shown in Figure 20. The dark lines repre- 
sent rods which are made of brass, while the 
other ones represent 
rods of steel. By 
looking at the figure 
you will see that the 
steel rods make the 
pendulum longer 
when they expand, 
and the brass rods 
make it shorter when 
they expand. The 
lengths of brass and 
steel are so calcu- 
lated that whenever 
the steel rods let the 
bob down the brass 
rods lift it up the 
Figure 19. — A Thermostat Installed: same amount. This 




BALANCE WHEEL OF A WATCH 



33 



13 



keeps the pendulum at the same length, regardless of the 
temperature. Another method of accomplishing the same 
thing is shown in Figure 21. 

The pendulum has a cup at the 
bottom, containing mercury. As the 
temperature rises, the rod of the 
pendulum becomes longer ; but at 

the same time 

the mercury ex- 
pands and rises 

in the cup, thus 

counteracting 

the expansion 

of the rod. 
37. Balance 

Wheel of a 

Watch. — Good 

watches have to 

be so made that 

change of tem- 
perature will 

not affect them. 

The balance 

wheel is to the 





Figure 20. — A Compen- 
sating Pendulum with 
Brass and Steel Rods. 



Figure 21. — A Compen- 
sating Pendulum with 
a Mercury Well. 



watch what the pendulum is to a 
clock. If the wheel gets larger, the 
watch runs more slowly; and vice 
versa. The rim of the wheel (Figure 
22) is made of two metals, steel and 
brass, just as is the pointer of the 
thermostat. The brass is put on the 
outside of the rim ; so that, when 



34 



EXPANSION 




Figure 22. — Balance Wheel of 
a Watch. 



the temperature rises and the spoke gets longer, the brass 

expands faster than the steel and makes the rim curve 

more, tending to make the 
wheel smaller. These two 
effects exactly counterbalance 
each other, and so the watch 
keeps even time. 

38. Hot Water Dangerous 
to Glassware. — Each of us 
has probably broken glass- 
ware by putting hot water 
into it. Why does hot water 
break the glass into which it 
is poured? Unequal expan- 
sion is the cause. 
As the water goes into the glass the inside is heated first, 

and so expands ; while the outside does not. This puts the 

glass under a great stress, and so it breaks. 

You feel the same effect in your teeth when you take a 

bite of ice cream or drink ice water. The outside of the 

teeth is cooled and contracts before the inside can cool off; 

and so the nerves are squeezed under a high pressure. 

If glasses are put into a pan of water and brought slowly 

to a boil, they will not break; nor will a very thin glass 

break as easily as a thick one when filled with hot water. 

Explain. 

When glass stoppers stick, they can often be gotten out 

of bottles by applying a flame to the neck of the bottle for 

a short time. This causes it to expand and so loosens the 

stopper. 

Thrusting the neck of the bottle into warm water will 

produce the same result. 



EXPAXSIOX EFFECTS WHEN WATER IS HEATED 35 

39. Coefficient of Cubical Expansion. — When a body is 
heated, it gets larger in every direction. Therefore it has 
more volume. This increasing in volume is called volume 
expansion. The coefficient of volume expansion is the 

increase in volume per degree C, per unit volume. 

Since a body expands in three directions, its coefficient 
of volume expansion is approximately three times its coeffi- 
cient of linear expansion. 

For example : What is the increase in volume of 1000 cubic centi- 
meters of aluminum for a range of 50° C. ? 

The coefficient of linear expansion for aluminum is .000022 ; so the 
coefficient of volume expansion is .000022 X 3 = .000066. 
Then 1000 X .000066 X 50 = 3.3 c.c. 
Therefore the 1000 c.c. of aluminum will increase 3.3 c.c. ; 
or will then contain 1000 + 3.3 = 1003.3 c.c. 

Problems 

1. Find the increase in length of an aluminum bar 60 cm. long 
when it is heated from 22° C. to 100° C. 

2. If an iron steam pipe leading from the boiler in the basement 
to an upper story room is 120 ft. long, and 20° C, how much will it 
expand when steam at 100° C. is passed through it? 

3. Will the lids fit tighter when the stove is hot or when it is cold? 
Why? 

4. If the pointer of a thermostat is 2" long, and is made of brass 
and steel, what is the difference in length of the brass and steel when 
it is heated 10° C? 

6. How much will a copper wire 10 ft. long expand in length if 
heated from 60° F. to 180° F. ? 

6. How much will 6000 c.c. of brass expand when heated from 
32° F. to 212° F. ? 

7. Will a glass flask hold more when hot or cold? Why? 

40. Peculiar Expansion Effects when Water is Heated. — 
Nearly all of our common substances expand when heat is 



36 ' EXPANSION 

applied, regardless of their state and temperature. For 
example a piece of iron will expand when heated ; and when 
it melts, it still expands; and when the molten metal is 
heated, it still expands; and likewise when it is vaporized 
and the gas is heated. Expansion takes place whenever 
heat is applied. 

But there is an exception to this rule. The exception is 
when ice is melting, and when the water is heated from 
0° C. to 4° C. 

If a piece of ice at a temperature below 0° C, say — 10° C, 
is heated, its temperature rises to 0° C, and the ice increases 
in volume. Then, if more heat is applied, the ice melts, the 
temperature remaining at 0° C. ; but the volume decreases. 
After it is all melted, the temperature again rises ; and until 
4° C. is reached, the water still contracts. After 4° C. is 
reached, the temperature continues to rise to 100° C, but 
the water expands. At 100° C. the water changes to steam, 
the temperature remaining at 100° C. until it is all steam; 
and the volume increases to about 1650 times its former 
volume. If, after the water is all steam, it is still heated 
at constant pressure, the temperature increases, and the 
gas expands. 

The best way to remember all this is to keep in mind 
that water is like all other common substances and expands 
when heated, except when melting and being raised from 
0° C. to 4° C. 

41. Importance of 4° C, the Temperature at which Water 
is Densest. — Did you ever think why the rivers and 
lakes freeze on top instead of at the bottom? The reason 
is that water is densest, — or, in other words, heaviest, 
per cubic unit, — at 4° C. 

In the summer time the temperature of the water may 



WHY WATER PIPES BURST 37 

reach 18° C. or 20° C. As the weather gets cooler in the 
fall, the top layers of water are cooled by the air. They 
are then heavier than the layers below them ; so they sink 
until they come to water as cool as, or cooler than, they are. 
This leaves exposed to the air a new layer which in turn 
cools and sinks. 

This displacement is kept up until the whole body of 
water is cooled to 4° C. Then, when the top layer gets 
colder than 4° C. it expands, and becomes lighter than the 
water below it ; therefore, it remains on top, continuing to 
get colder and lighter. When it reaches 0° C, it freezes 
and expands still more. This ice layer protects the un- 
frozen water, which remains at 4° C, except for the layers 
next the ice. 

If water were like mercury and continued to contract as it 
cooled, large bodies of water would freeze solid in cold 
weather. The water would cool at the top and sink, letting 
the warmer water come to the surface. This would con- 
tinue till all the water was at the freezing point, when the 
top would begin to freeze. Then the ice would sink; and 
the lakes and rivers would be frozen from the bottom up. 
In a cold winter they would be a mass of solid ice. 

Then in the summer the ice would melt only on top, 
leaving the lake almost a solid cake of ice. The result 
would be a climate too cold for vegetable life. 

42. Why Water Pipes Burst. — When water is allowed 
to remain in the water pipes in very cold weather, it freezes 
and expands, thus breaking the pipes. The ice acts as a 
plug in the pipe, otherwise the expansion would force the 
water back into the water mains, in which case the pipes 
would not break. It is because the water is imprisoned in 
the pipe behind the ice plug that the pipe must give way. 



38 EXPANSION 

43. Expansion of Gases. — We found, from our study of 
expansion of liquids and solids, that they all expand at a 
different rate, making it necessary to have a table of 
coefficients of expansion. In the case of gases this 
is different, all gases expanding at the same rate. There- 
fore there is only one coefficient of expansion for all 
gases. 

If a certain volume of gas be heated 1° C, it will expand 
2+3 of its volume at 0° C, if kept at the same pressure. 
This fraction, 2T3, or .00366, is the coefficient of expansion 
of gases. 

If 273 c.c. of oxygen, hydrogen, air, or any other gas, 
were heated from 0° C. to 1° C, the gas would expand 
2-73 of 273 c.c. = 1 c.c. Therefore the same amount of 
gas would fill a vessel of 274 c.c. at the new temperature, 
the pressure remaining the same. 

44. Absolute Zero. — Gases, like all substances, are com- 
posed of molecules; but under normal pressure and tem- 
perature the molecules are comparatively far apart. It 
has been said that if the molecules of a gas, such as ordinary 
air, were magnified until they were the size of an orange, 
each molecule would be surrounded by a space equal to a 
cubic yard. If this is true, the space actually taken up 
by the molecules is very small, and the empty space about 
them is large. 

When heat is applied, each molecule flies faster than 
usual, bumping its neighbors farther apart, thus making 
the space about it larger. If the gas is cooled, the mole- 
cules move more slowly than usual ; and consequently come 
closer together. The more the gas is cooled, the more slowly 
the molecules move, until, theoretically, they come to rest. 
There is then absolutely no heat in the gas. When at rest 



SOME APPLICATIONS OF CHARLES' LAW 39 

they occupy so little space that it is not counted at all; 
and the gas is said to have no volume. 

The temperature at which a gas has no volume is 
— 273° C. This temperature is then called absolute zero, 
because it means total absence of heat. 

45. Charles' Law. — A man by the name of Charles 
formulated a law about the expansion of gases. This is 
called Charles' Law : 

' The volume of a gas at constant pressure is proportional 
to its absolute temperature." 

Example : What is the volume of a gas at 70° C, if it 
occupies 800 c.c. at 20° C. ? 

Solution : The original absolute temperature is 20 + 273 = 293° ; and 

the final absolute temperature is 70 + 273 = 343°. Since, by Charles' 

Law, the volume of a gas is proportional to its absolute temperature, 

the new volume is Iff of 800 c.c. = 936.5 + c.c, or 

7 new absolute temperature v , • • , 7 

new volume = ±- X original volume. 

old absolut > temperature 

46. Some Applications of Charles' Law. — The expansion 
of gases has much to do with the baking of bread, cake, or 
pie. 

To make bread, yeast is used to produce the rising. The 
dough is mixed and allowed to stand in a warm place. The 
yeast plants grow and, in growing, give up carbon dioxide 
gas. The dough does not allow this gas to escape; so it 
forms bubbles in the dough, causing it to rise. The dough 
is then " worked down," and again allowed to rise in the 
same way. Usually it is " worked down " a second time 
and again allowed to rise. When it has risen properly, it 
is placed in a hot oven and baked. 

Lp to this time the rising has been caused by the growing 
yeast plants. But when it is put into the oven, the heat 



40 EXPANSION 

kills the yeast plants; so the rising after that is due to 
something else. The carbon dioxide bubbles in the dough 
are heated. According to Charles' Law, they expand 2T¥ 
of their volume at 0°C. for every degree Centigrade they are 
raised in temperature. This makes the bread rise while 
it is baking. 

In baking biscuits and cakes, baking powder is used 
instead of yeast. But the action is the same. Baking 
powder, when wet, gives off carbon dioxide. The rising 
takes place as in the case of the yeast. Expansion also 
takes place when the cake or biscuits are placed in the oven. 

In making pie crust there is usually nothing put into the 
dough to make it rise. But the crust must rise a little ; or 
else it will be tough, instead of brittle and flaky. The ex- 
pansion of gases is used to produce this rise. In mixing, 
the dough should be worked very lightly and the flour 
should be sifted in. Doing this gets air into the dough, 
and the light working leaves it there. Then if the dough 
is chilled by placing it in the refrigerator, the open spaces 
will fill up with cold air. This cold air will expand when 
the pie is baked, producing a brittle, flaky crust. 

On the other hand, in clay modeling care is taken to work 
all the air out. The clay is kneaded and pounded and 
squeezed so that no air is left in it. If the air is not all out, 
when the piece is fired in the kiln these bubbles expand and 
break the piece of pottery. 

Other applications of the expansion of gases, which will 
be studied under another topic, are : the draft in a stove, 
grate, furnace, chimney, range; hot-air heating; and 
ventilation. 



CHAPTER III 
HEAT TRANSFERENCE 

47. Transference of Heat. — Heat is transferred from 
one place to another by three methods, conduction, convection, 
and radiation. Each of these will be taken up in detail. 

48. Conduction. — If heat is applied to one part of a 
body, the molecules will be set into rapid vibration at that 
point. These molecules strike their neighbor molecules 
and set them in vibration. These in turn set the next ones 
going, and the heat travels along the body by conduction. 

If one end of a poker is placed in the fire, that end gets 
hot, and all the rest of the poker is warmed. But the 
temperature is lower, the farther away from the end in the 
fire. 

Different materials conduct heat at different rates. Those 
that conduct it very readily are called good conductors. 
Those that do not conduct heat readily are poor conductors, 
or are good insulators. Silver, copper, gold, aluminum, iron, 
and nearly all other metals are good conductors. Among 
the poor conductors, or good insulators, are asbestos, a 
vacuum, air space, water, paper, wood, glass, cloth, por- 
celain, horn, and ivory. 

49. Non-conducting Handles for Cooking Utensils. — 
Figures 23, 24, 25, and 26 show different methods used to 
keep the handles of cooking utensils cool. The teakettle 
is made of metal, all except the handle, and that is made 

41 



42 



HEAT TRANSFERENCE 



of wood. The metal becomes hot by conduction, but the 
wood does not let the heat through. 

The coffee-pot and the percolator have handles of wood, 
porcelain, horn, or ivory, for the same reason. The stove- 
poker has a metal 
handle, but it consists 



Wood 



Porcelain, Horn or Ivory 





Figure 23. — Wood Handles 
on a Tea-kettle. 



Figure 24. — The Handles of the 
Coffee-pot are Insulated. 



Glass 



Horn or Wood 



of a wire wound in a coil about the end of the poker. 

This allows air space between the poker and the wire 

handle, and this air space is a good insulator. 

50. Good Conductor Bottoms on Utensils. — The bottoms 

of coffee-pots, tea-kettles, wash-boilers, etc., are usually of 

copper. This is for two 
reasons. First, copper will 
not corrode as readily as iron 
or tin; and therefore will 
keep cleaner and last longer. 
Second, copper is a good con- 
ductor, so that the heat is 
readily conducted from the gas 

Insulation for the flame 0r from the stoVe to P to 
Handles of a Percolator. the contents of the utensil. 




THE FIRELESS COOKER 



43 



51. The Fireless Cooker. — The fireless cooker is a box 
arrangement with non-conducting walls. Figure 27 shows 
how it is constructed. /CoilofWire 
On the inside are pails in 
which the food is placed. 
Around the pails is the 
non-conducting wall. The 
food is first heated to the 
boiling point, and at the 
same time slabs of soap 
stone or iron are heated. 
When these are hot 
enough, the hot food is placed in the pails between the 
hot slabs; then the whole box is closed up tight. 

The non-conducting walls keep the heat in, so that the 
food stays up close to the boiling temperature without 




Figure 26. — Coiled Wire Handle on 
a Stove-poker. 



Pail of Food 




Hot Plate 
Wood 

Felt 

Asbestos 

Enamel Ware 
Mineral Wool 



Figure 27. — The Fireless Cooker. 



being supplied with more heat. This makes it necessary 
to use the fire only long enough to get the food and heating 
slabs hot. 



44 



HEAT TRANSFERENCE 



The non-conducting material used may be wool, felt, 
mineral wool, asbestos, leather, paper, straw, shavings, or 
sawdust. 

52. The Refrigerator. — The refrigerator (Figure 28) 
uses non-conducting substances for its walls. On the out- 
side is usually wood; next is an insulating layer of paper; 
then another of wood ; then a layer of asbestos or felt ; 
and then an air space. The inside material is usually glass, 



Glass or 
Enamel Ware 



Air Space 




Mineral Wool or Felt 

Rough Wood 
Paper or Asbestos 
Finished Wood 



Figure 28. — The Cross Section of a Refrigerator Wall. 



enamel, or zinc. The ice is put into the top of the refrigera- 
tor, and the things to be kept cool on the shelves below or 
beside it. The insulating walls allow little heat to come 
in from the outside ; so that most of the heat used to melt 
the ice comes from the articles put in to be cooled. 

53. The Thermos Bottle. — The thermos bottle consists 
of a double glass flask with the outside silver-coated (Figure 
29). The space between the walls of the flask is a vacuum, 
the air having been pumped out. The flask is then placed 
inside of an outer cover, which is either silver or nickel 



WALLS OF HOUSES 



45 



Air Space 
Metal Case- 




^ 



Screw Cap 
Cork 

Glass Flask 
Vacuum 



Contents 

Where Glass Flask 
Was Sealed 



plated. An air space is left between the outer cover and 
the glass flask. 

The bottle is used to keep liquids either cold or hot. 
When cold liquids are placed in it, the heat is kept out by 
the insulating walls ; and if hot liquids are placed in it, 
the insulating walls keep the heat in. 

The reasons for this are apparent. First, the glass walls 
of the flask are non-conductors, and do not permit heat to 
pass through them 
easily. Then, the 
vacuum is the best 
non-conductor there 
is. Also, the air 
space between the 
outside cover and the 
flask helps the in- 
sulation. Finally, 
the silvered and nickled surfaces have special uses, which 
will be discussed under the subject of Radiation. 

Good thermos bottles will keep coffee too hot to drink 
for fifteen hours. Care must be taken to have the liquid 
hot when it is placed in the bottle. , 

54. Walls of Houses. — Walls of houses are so con- 
structed that they do not allow the heat to pass through 
them readily. Either brick, stone or lumber is used. 
The lumber-made house is constructed as shown in 
Figure 30. 

First is put up studding, which is about two inches by 
four inches. On the outside of this is nailed rough lumber, 
called sheathing. Over this is usually tacked heavy paper, 
and then the siding or weather-board. Inside the studding 
the plaster lath is nailed, and then the plaster is spread 



Figure 29. — Cross Section of a 
Thermos Bottle. 



46 



HEAT TRANSFERENCE 



over this. This constitutes the complete wall, except for 
the wall paper usually placed on the inside. 

Naming the insulating layers from the outside inward, 
they are, weather-board, heavy paper, sheathing, air space, 
plaster lath, plaster, and wall paper. 



Studding 



Lath 



Plaster 



Wall Paper 




Sheathing 



Figure 30. — Cross Section of the Wall of a House. 



Sometimes in cold countries an extra set of lath and 
plaster is put in between the studding, making also an extra 
air space. 

55. Clothes. — Winter clothing is usually made of non- 
conductors. We wear light cotton clothes in summer and 
heavy woolens in winter. Why? The cotton is compact 
and conducts heat readily, while the wool is loose in con- 
struction, containing many air spaces, which act as insulators. 
You can easily tell the difference between cotton and wool 
by dampening the thumb and finger and rolling a thread 
of each between them. The cotton will pack closely to- 



CONVECTION 



47 



gether, while the wool will spring back to its original loose- 
ness. 

56. Convection. — Convection is the second method of 
transferring heat. In conduction we learned that it was 
the heat energy only that moved along. In convection, 
the heat passes from one place to another by means of 
material bodies carrying it. 

Convection can best be understood by studying the 
following drawing. Figure 31 shows a section of air divided 
into columns. If a r , 



£> 



\ 



A 



T t 
t f 

t t 

i t 
; t 



B 



X 



Figure 31. — Diagram Showing how Con 
vection Currents are Set Up. 



fire were built under 
column A BCD, the 
air would be heated 
and would conse- 
quently expand. As 
the air cannot push 
sidewise, on account 
of the other columns 
of air, when it ex- 
pands it must push 
upward. This makes this column higher than the others ; 
so the air flows outward over the other air columns at the 
top, as indicated by the arrows. 

Now this makes the columns at the side heavier than 
the middle one; so they crowd down, forcing some of the 
cold air under the middle column, as indicated by the ar- 
rows. This air will then be heated, will expand, and be 
pushed up by more cold air. 

So the process goes on ; the cold air flowing towards the 
warm area at the bottom, and the warm air flowing away 
from the warm area at the top. Over the warm area the 
air moves upward, while over the cold area the air moves 



48 



HEAT TRANSFERENCE 




downward. These movements are called convection cur- 
rents. 

Convection currents take place in liquids as well as in 
gases, but cannot take place in solids. 

57. Drafts in Chimneys. — Drafts in chimneys are due 
to convection currents. A fire is started in the fire-box of 
the furnace. (Figure 32.) This warms 
the air, and causes it to expand and 
become lighter than the surrounding air. 
The cold air then pushes the warm air 
up the chimney and takes its place in 
the fire-box. This air is then heated, 
and the process is repeated, or rather it 
takes place continuously. The higher 

the chimney, the greater the draft. 

Figure 32. — Draft 

in. a Chimney. Suppose the chimney (Figure 32) were 4 ft. 

square and 100 ft. high; and suppose the air 
raised from 0° C. to 273° C, when the fire started. 

4 X 4 X 100 = 1600 cu. ft. = volume of the chimney. 

Now, air at 0° C. weighs .08 lb. per cu. ft. 

1600 X -08 = 128.00 lb. = wt. of air in the chimney, when air is 
cold. 

Since a gas expands 273 of its volume at 0° C. when heated 1° C, 
it will double its volume when heated to 273° C. 

Therefore, since the chimney will contain only 1600 cu. ft., \ of the 
air must flow out. 

\ of 128 lb. = 64 lb., wt. of air which remains in the chimney. 

Now, since an equal volume of air on the outside weighs 128 lb., 
and inside it weighs 64 lb., the cold air outside pushes up on the 
warm air inside with a force of 64 lb. This shows definitely why the 
air rises in the chimney, or explains the draft. 



58. Draft in a Kitchen Range. — Figure 33 shows the 
ordinary kitchen range. The air enters at the front and 



DRAFT IN A KITCHEN RANGE 



49 



Damper 




Figure 33. — Draft in a Kitchen Range. 

goes up to the fire-box. Here it becomes hot and, with the 
smoke, passes up over the oven, down at the end and under 




Figure 34. — Diagram of a Hot-air Heating System. 



50 



HEAT TRANSFERENCE 



the hot-water reservoir, then under the oven, and finally 
up, at the back of the oven, to the stove pipe. Thus we 
see the hot gases pass completely around the oven, except 
in front, where the door is located. 

If the oven is not to be used, the damper is closed, which 
shuts the current off from the oven and lets the hot gases 
circulate only under the top of the stove and the reservoir. 

59. Hot-air Heating. 
— Figure 34 shows a 
diagram of the modern 
hot-air heating system. 
The furnace located in 
the basement consists 
of two parts, a fire- 
box, and a sheet-iron 
jacket, the two being 
separated by an air 
space. 

The air that feeds the 
fire in the fire-box goes 
in through a hearth, and 
the smoke and gases 
pass on up the chimney. 
This air and other gases never reach the rooms, nor are they 
even in contact with the air that goes to the rooms. The 
latter comes in through the cold-air shaft (from outside or 
from the basement itself) ; is heated as it passes between 
the sheet-iron jacket and the wall of the fire-box; then is 
carried in convection currents through pipes that lead to 
the separate rooms. 

60. Hot-water Tank. — Convection currents take place 
in liquids as well as in gases. Use is made of this in the 




Figure 35. — A Hot-air Furnace, 



HOT-WATER HEATING SYSTEM 



51 




hot-water tank. Figure 36 shows a hot-water tank designed 
to be heated by a separate heater, or by the furnace itself. 

The water comes into the storage tank (A) through 
pipe (/). A pipe (g) comes out of the storage tank at the 
bottom and passes up through a pipe (i), around which is 
the heater (B). This pipe then returns to the top of the 
tank through (h). The pipe (c) is for drawing off the hot 
water to the places 
where it is needed. 

A fire is started in 
the heater (B), causing 
the water in pipe (i) to 
expand. Convection 
currents are then set 
up, and the warm 
water flows over into 
the top of the tank, 
cold water coming in 
all the time at pipe (g). If the furnace (0) is going, the 
heater (B) is not needed, as the convection currents are 
set up through the coils in the furnace. When water is 
drawn off through (c), more water is supplied through the 
inlet, from the water main. 

If the water is allowed to get too hot, steam is generated, 
which may force the water back into the main, thus en- 
dangering the water meter. 

61. Hot- water Heating System. — Figure 38 shows a 
modern hot-water heating system. The furnace is located 
in the basement, and has a boiler above the fire-box. From 
the top of the boiler, pipes are led off to the radiators in the 
different rooms. Returning from the other end of the 
radiators are pipes to bring the water back to the bottom 



Figure 36. — Diagram of the Heating 
System of a Hot-water Tank. 



52 



HEAT TRANSFERENCE 



of the boiler. The pipes going up to the radiators are 
called " risers," while those coming down are called " return 
pipes." Connected in the system is a pipe which goes up 
to the expansion tank, usually placed in the attic. 

' j>hi— ■» iiii I ii ii , ■ yAiiiHiw^ 




Figure 37. — A Kerosene Heater Used in Connection with thi 
Hot- water Tank. 



HOT-WATER HEATING SYSTEM 



53 



Before the furnace is started, water is let in from the city 
main until the whole system is full and water rises into the 
expansion tank. Then the stop-cock is closed, so that no 
more water can get in or out. When the fire is started, 




Water 
Main 



Figure 38. — Diagram of a Hot-water Heating System. 



convection currents are set up through the pipes, causing 
hot water to flow through the radiators. 

The expansion tank is to protect the pipes from bursting. 
If there were no place for the water to go when the fire is 
started, the expansion would burst the boiler or the pipes. 
This sometimes happens if the pipe to the expansion tank 
in the attic freezes. 



54 



HEAT TRANSFERENCE 




Figure 39. — A Hot-water Heating System Installed. 



62. Ventilation. — Ventilation is the supplying of pure 
air and the removing of impure air from rooms and 
buildings. 



VENTILATION 



55 



It is estimated that every person should have 3000 cubic 
feet of pure air per hour. There are two distinct types of 
ventilation — the natural systems and the forced systems. 

In the natural systems convection currents are depended 
upon to change the air. In many dwelling houses no special 
means are used for ventilation ; open windows, doors, or 
crevices are depended upon entirely to supply pure air. 

If a window is opened both 
at the top and bottom, as is 
shown by Figure 40, and a 
lighted candle is held, first 
at the bottom, and then at 
the top, of the window, the 
candle flame will blow to- 
wards the room in the former 
position, but will blow out- 
wards when held at the top, 
showing that air enters at the 
bottom and leaves at the top. 
This is explained by convec- 
tion currents. Opening win- 
dows is a quick means of 
getting ventilation, but it 
produces drafts. 

Even when the windows or doors are closed, air comes 
in around the frames, where there is not a perfect fit. This 
supplies pure air and is sufficient in many cases where very 
few people use the rooms. Wind coming from one side of 
the house often helps ventilate it, blowing pure air in on 
one side and forcing impure air out on the other. 

A grate or fireplace is a good ventilator. Why? 

One of the simplest methods for special ventilation is 



Outside 




Inside 



Figure 40. — Ventilation by Means 
of the Open Window. 



56 



HEAT TRANSFERENCE 



in 



ft 



Air Duct to Outside 



shown in Figure 41. A cold air vent is made just below the 
radiator. As the cold air comes in, it is heated by the radia- 
tor and made to flow to all parts of the room by means of 
convection currents. The impure air 
leaves by way of crevices. 

Another of the natural systems is 
shown in Figure 42. Here the air 
comes in from the outside, passes 
around a special heating device in the 
Figure 41. -Another fl 0OTj an d then is distributed by con- 

Method of Ventila- . 

TI0N vection currents. 

Forced ventilation is used in large 

buildings, such as schools, apartment houses, department 

stores, and theaters. In such buildings there are great 

numbers of people, and the ordinary method of ventilation 

is not sufficient to supply the required 3000 cubic feet, per 

hour, for each person. 

Air-duct to Outside 
r orced- ventilation 

systems use fans to 
make the air move. 
One way is to draw the 
impure air out by means 
of fans, allowing the 
pure air to flow in to 
take its place. Other 
methods force the pure 
air in, driving the im- 
pure air out. 

Figure 43 shows a 
forced-ventilating system in which the air is washed before 
it passes through the rooms. Pure air, forced in by the fan, 
enters the washing room. The washing room consists of a 




Figure 42. — Ventilation with Heating 
Device in the Floor. 



RADIATION 



57 



Heating Room 



closed space in which water is kept spraying. Here the air 
has most of the dust and impurities removed. Then it is 
forced up the pipes to the heating space, and from there 
it goes to the places where it is needed. 

63. Radiation. — Conduction and convection, the two 
methods of transference of heat which we have just studied, 
are easily understood ; but the third method, radiation, is 
much more difficult to 
explain. We know that 
heat travels from the 
sun to the earth, and 
that it comes through 
space in the form of 
waves in the ether. 

No one knows just 
what the ether is, but 
there are many facts 
which prove its exist- 
ence. Whatever it is, 
it has no weight or 
body, but it fills the 
whole universe. 

Heat in the form of 
waves in the ether is 
insensible, for sensible heat is due to the vibration of 
molecules. 

When heat waves strike opaque objects, they are partly 
changed to sensible heat and partly reflected back as waves. 
When they strike transparent objects, such as air, glass, clear 
water, etc., they pass through without heating the object. 

Radiation is the transference of heat by means of waves in 
the ether. 




Washing Room 



Figure 43. — Diagram of a Forced- 
ventilating System. 



58 HEAT TRANSFERENCE 

64. Radiators. — We must not get the idea that the sun 
is the only thing that sends out these heat waves, for all 
hot bodies do this, more or less. Any body that sends out 
heat waves is called a radiator. 

All bodies at the same temperature do not radiate their 
heat at the same rate. It is found that rough, black bodies 
are the best radiators, while smooth, white, or shiny objects 
radiate heat very slowly. 

65. Absorbers. — Heat waves striking opaque objects 
are changed to sensible heat. These objects are said to 
absorb the heat waves. Bodies which are good radiators, 
namely, rough black ones, are also good absorbers. A 
rough, black piece of iron will cool off quickly after it is 
heated, because it is a good radiator; and, on the other 
hand, it will become warm quickly if placed where heat 
waves fall on it, because it is a good absorber. 

66. Reflectors. — Why are rough, black objects good 
radiators and good absorbers, while smooth, white, or shiny 
objects are poor ones ? The answer is that smooth, white, 
or shiny objects are good reflectors. The heat waves fall on 
them and are reflected back, just as light is reflected by a 
mirror. On the other hand, when the heat waves start to 
leave the objects, the shiny surface turns them back again. 

67. Applications. — In the thermos bottle (§ 53) the 
glass and the vacuum stop conduction and convection, but 
cannot stop the heat from radiating into or out of the bottle. 
This is stopped by the silver surfaces. As they are smooth, 
and shiny, any heat trying to radiate into the bottle is re- 
flected out again ; and any heat trying to radiate out is 
reflected in again. Therefore all three avenues for the 
transference of heat are stopped, so that either hot or cold 
liquids put into the bottle remain hot or cold. 



APPLICATIONS 59 

A black, rough stove would be more serviceable than a 
bright, shiny one. Why? What kind of clothes would 
you wear in hot weather or in a warm climate? In a cold 
climate ? Why ? 

Greenhouses trap the heat of the sun and do not let it 
out. The heat waves pass through the glass of the green- 
house and strike the plants and soil and other objects, 
which absorb the waves. In other words, the waves are 
changed to sensible heat. The glass walls are poor con- 
ductors ; so the sensible heat cannot get out. 

Dirty snow does not melt evenly, but in holes and patches. 
Soot and dirt, being black, absorb the sun's rays and thus 
melt the snow under them, causing holes in the snow. Where 
there is no dirt, the snow reflects the rays and therefore 
melts more slowly. 

On a sunny day, would the snow melt faster under a 
black woolen blanket, or without the blanket? Would it 
be the same by night, or if the day were cloudy ? 



CHAPTER IV 
SOURCES OF HEAT 

68. Fuels. — We have studied the nature of heat, have 
seen what it will do, and how it is transferred from one 
place to another. Now comes the question, where do we 
get heat ? 

The sun is the great source of heat, but the sun's heat 
is so widely distributed and so little under our control, that 
it serves mostly the processes of nature, and for specific 
purposes of service we rely mainly on fuels. 

Fuels are materials which will supply heat when burned. 
Sixty years ago the most common fuel was wood. What 
fuel do you use at home to keep warm and to do your cook- 
ing? Most of you will say gas, or coal. 

There are two distinct kinds of gas — natural gas and 
artificial gas. The natural gas is forced directly from the 
gas well to your home. The artificial gas does not come 
from wells at all, but is made by baking soft coal and treat- 
ing it in certain ways. 

Natural gas is much better for heating purposes than 
artificial gas, since the natural gives 1200 B. T. U.'s per cubic 
foot, while artificial gas gives only half as much, or 600 
B. T. U.'s per cubic foot. 

There are many kinds of coal, but we usually speak of 
two, hard and soft The hard coal is " clean," that is, it 
has little dust in it and gives off little smoke when it burns. 

60 



FUELS 



61 



The soft coal is full of dust and its smoke is dense and 
sooty. 

Hard coal yields about 14,000 B. T. U.'s per pound, when 
burned; while soft coal yields about 12,000 B. T. U.'s per 
pound. It is never possible to get all the heat when a fuel 




Figure 44. — Kerosene Used as a Fuel in the Cook Stove. 



is burned, but more is available in some fuels than in others. 
This is true of coal. Hard coal would give only about 2000 
B. T. U.'s per pound more than soft coal, if one could get all 
the heat. But a great deal more heat is lost in the case of 
soft coal than in the case of hard coal ; so that, in the end, 
hard coal heats much better than soft coal. 

The following table gives a few of the materials used 



62 



SOURCES OF HEAT 



for fuels, and the name or kind of each. Opposite each kind 
of fuel is the heat value. 



Sources of Heat 



Material 


Kind 


Heat Value 


Coal 


[Hard 


14000 B.T.U.'s per lb. 


^ Soft 


12000 B.T.U.'s per lb. 




[ Coke 


14000 B.T.U.'s per lb. 


Wood 


/Hard 

[Soft 


8400 B.T.U.'s per lb. 




8600 B.T.U.'s per lb. 


Gas 


/ Natural 
[ Artificial 


1200 B.T.U.'s per cu. ft. 


600 B.T.U.'s per cu. ft. 




[ Kerosene 


20000 B.T.U.'s per lb. 


Oils 


{ Naphtha 
[ Crude Oil 


20000 B.T.U.'s per lb. 




18000 B.T.U.'s per lb. 


Electricity .... 




3411.72 B. T. U.'sper Kw. hr. 



(Electricity is given in this table, though it is not a fuel.) 

69. The Gas Meter. — The gas that you use is measured 
by a gas meter. The gas, flowing through the meter, moves 
little fans, making the hands move around on the dials. 

1,000,000 100,000 10,000 1,000 




2 6 3 4 

Figure 45. — Dials of a Gas Meter Showing a Reading of 
263,400 Cu. Ft. 

These dials indicate how much gas has passed through the 
meter. The figures above the dials indicate the number of 
cubic feet that have passed when the hand makes one com- 
plete revolution. 



HEAT FROM FOODS 63 

Figure 45 shows a four-dial meter with a reading of 263,400 
cu. ft. 

Always begin to read from the right-hand side. 

Your gas bill is made out from these meter readings. 
The meter man comes round every month and reads the 
meter. The last month's reading is subtracted from the 
present month's reading, and the number of thousand (M) 
cubic feet of gas used during the present month is thus deter- 
mined. Only integral numbers of thousand cubic feet are 
counted. Thus, if the meter reads 263,400 cu. ft., the 400 
is not counted, but the reading is called 263 M. 

The cost of natural gas in Cleveland at present is 30^ 
per M. while that of artificial gas is 80^ per M. 

Problems 

1. How much hard coal is necessary to melt 150 lb. of ice when 
12 per cent of the heat is available ? 

2. How much soft coal is necessary to heat 150 lb. of water from 
40° F. to 100° F., only 6 per cent of the heat being available? 

3. What will be the cost of the natural gas required to boil 10 lb. 
of water away, if 10 per cent of the heat is available? Natural gas 
costs 30^ per M. 

4. How many B. T. U.'s are given off when a ton of soft coal is 
burned ? 

5. What is the cost of boiling away 10 lb. of water, if artificial gas 
is used at 80^ per M? 

6. Draw a 4-dial gas meter showing a reading of 267,300 cu. ft. 

7. WTiat is the month's natural gas bill if the meter read 246,300 cu. 
ft.' last month and 252,600 cu. ft. this month ? 

70. Heat from Foods. — The energy we use in the body 
comes from the foods we eat. In other words, our food is 
fuel. Part of the food is used for building and repairing 
tissue, but certain kinds are for fuel. 

The United States Government has made charts of the 



64 SOURCES OF HEAT 

building value and the heat value of most of our foods. A 
study of these charts is to be made at this point. 

An average laboring man should have from 3000 to 
3500 great calories of heat per day. A person not at manual 
labor should have less — it is estimated about 2500 great 
calories. 

From the table in the Appendix, make up a day's menu 
so that the person shall get about 2500 calories. Figure the 
cost of each item and make a total for each meal. Calcu- 
late the cost for the whole day. 

Review Problems 

1. What is the nature of heat ? 

2. What is meant by the terms hot and cold? 

3. Define temperature. 

4. Change 25° F., - 16° F., 75° F. to the corresponding Centi- 
grade readings. 

5. Change 10° C, — 8° C, 80° C. to the corresponding Fahrenheit 
readings. 

6. Define freezing point ; boiling point. 

7. Explain the effect of pressure on the freezing point; on the 
boiling point. 

8. Name and explain two applications of the effect of pressure 
on the boiling point. 

9. What are the three heat units used ? Define each. 

10. Discuss heat of fusion. 

11. Discuss the refrigerator as an application of heat of fusion of 
water. 

12. Discuss heat of vaporization. 

13. Discuss the double boiler as an application of heat of vaporiza- 
tion of water. 

14. Explain distillation. 

16. What is meant by " iceless refrigeration " ? 
16. How many calories are necessary to melt 20 kg. of ice without 
changing its temperature? (One kg. = 1000 grams.) 



REVIEW PROBLEMS 65 

17. How many B. T. U.'s are necessary to melt 50 lb. of ice ? Where 
does the heat come from if the ice is in a refrigerator ? 

18. If the ice on a lake one mile square is h foot thick, how many 
B. T. U.'s are necessary to melt it? (Assume that ice weighs 52 lb. 
per cu. ft. and is at 0° Centigrade.) 

19. How many B. T. U.'s are given off when 6 lb. of steam con- 
denses in the radiator ? 

20. Explain dew. 

21. Define specific lieat. 

22. Name and explain two applications of specific heat. 

23. Explain expansion. 

24. How much will a 40 cm. glass tube expand in length when 
heated 20° C? 

25. How much larger than the rest of the gla:s will the bottom 
of a two-inch drinking glass become when the bottom is suddenly 
thrust into boiling water (212° F.) ? (Assume that the original 
temperature was 80° F.) What will this expansion do to the 
glass ? 

26. Explain the thermostat. 

27. Why do water pipes burst when they freeze? 

28. "What is the volume coefficient of expansion of a gas? 

29. Explain the meaning of absolute zero. 

30. What application has Charles' Law to the baking of bread and 
cake? 

31. Explain conduction. 

32. Give three applications of conduction as a method of heat 
transference. 

33. Explain convection. 

34. Why does the smoke flow out of a chimney ? 

35. Explain how the water is heated in the hot-water tank. 

36. How long would the air in a room 20 ft. X 18 ft. X 10 ft. re- 
main healthful if five persons were in it ? 

37. What are the two types of ventilation ? 

38. Discuss radiation. 

39. Discuss radiators, absorbers, and reflectors. 

40. Name and explain three applications of radiation. 

41. What is a fuel? 

42. How much natural gas is necessary to heat 100 lb. of water for 



66 SOURCES OF HEAT 

a bath, if the water is at 38° F. at the beginning, and 100° F. when 
heated ? (Assume that 8 per cent of the heat is available.) 

43. How much soft coal is necessary to melt 50 lb. of ice, if only 
6 per cent of the heat is available ? 

44. What is the cost per gallon of distilling water, if natural gas 
is used and 10 per cent of the heat is available ? (Assume that the 
water has to be raised from 38° F.) 

45. Why should the food one eats have a certain heat value ? 



CHAPTER V 
WAVE MOTION 

71. Examples of Wave Motion. — Sound and light are 
the commonest examples of wave motion ; but the example 
most readily seen is the waves formed on water when some- 
thing disturbs its surface. If a stone is thrown into still 
water, a splash occurs at the point where the stone strikes, 
and waves travel outward in all directions from this point. 
If a cork, or anything that will float, is placed on the water, 
it is seen to bob up and down ; but it does not move away 
from its original position. 

A similar example is the waves produced in a field of 
grain when the wind blows over it. The individual heads 
of grain merely rise 
and fall, but the 
wave travels across 
the field. 

If a rope or rubber Figure 46. — Wave in a Rope. 

hose is held station- 
ary at one end and the other end is shaken, waves will be 
sent down the rope. (Figure 46.) The waves travel from 
one end of the rope to the other, but each particle of the 
rope, such as P, jumps up and down, but does not move 
forward. 

Figure 47 shows a spiral spring, attached to a support 
at the top, having its bottom suddenly jerked downward. 

67 




68 



WAVE MOTION 



Figure 47. — Wave 
in a Spring. 



A portion of the spring a is stretched, but the rest of the coil 
6 remains the same as before it was jerked. The next in- 
stant part a pulls down on part b and 
stretches b, letting a go back to its first 
position. 

This is a form of wave in which the 
waves move along the spring, and each 
particle of the spring jerks backward 
and forward, parallel with the spring. 
Waves can be sent along rubber bands 
just as along the spring mentioned above. 
Suppose a rubber ball is in the center 
of the room, fastened by rubber bands 
to all the walls, the ceiling, and the floor. 
(Figure 48.) Then suppose the rubber 
ball contracts suddenly. All the rubber bands next the ball 
will be stretched, as shown in Figure 49. Each stretched 
portion will, in turn, stretch the next portion ; and so on, 
until the effect runs out to the ends of all the rubber bands, 
just as it did in the spring. 
Since this effect travels out 
at the same speed in all the 
bands, we can think of it as 
being a wave like the wave 
on the water. 

72. Origin of Waves.— 
It is seen from all the pre- 
ceding examples that waves 
have to be started. This is 
always true. In the case of 

the water wave, the Stone Figure 48. -A Rubber Ball At- 

TACHED TO THE SlDES OF A ROOM 

Started the disturbance; in by Means of Rubber Bands. 




CHARACTERISTICS OF TRANSVERSE WAVES 69 

the field of grain, it was the wind ; in the rope, your hand 
was the cause. The same thing was true with the spring; 
and the contraction of the 
rubber ball started the wave I " ^ '( ^S) 



in the rubber bands. 

73. Transverse and Longi- Figure 49.— A Stretched Portion 

-.-_-- rm OF A RUBBER BAND NEXT THE 

tudinal Waves. — lnere are BALLt 
two motions in each case 

mentioned : the motion of the wave, and the motion of the 
particles of water, rope, spring, rubber, or grain heads. 

The relative direc- 
£. >tf tions of these two mo- 

* tions determine the 

Figure 50. — Showing Directions of -, . -, £ j 

Motions in a Transverse Wave. kmd of wave under 

consideration. Waves 

in which the particles move at right angles to the direction in 
which the wave moves are called transverse waves. (Figure 
50.) The long arrow W indicates the direction of the wave, 
and the arrow P indicates the direction in which the par- 
ticle moves. 

Waves in which the particles move parallel with the direction 
in which the wave moves are called longitudinal waves. 
(Figure 51.) Here the p 

two arrows are parallel, ^^ ->w 

and SO show a longi- Figure 51.- Showing Directions of 

Motions in a Longitudinal Wave. 
tudmal wave. 

74. Characteristics of Transverse Waves. — In case of 
the waves in the water, in the grain, and in the rope, we 
find that, as the waves follow one another, parts of the 
material are high and other parts low. The high parts a 
and c (Figure 52) are called crests, while the low parts b 
and d are called troughs. 



70 



WAVE MOTION 



The distance ac from one crest to a corresponding point 
in the next crest is called a wave length; or it may be from one 
trough to the corresponding point in the next trough, bd. 




Figure 52. 



b d 

Characteristics of a Transverse Wave. 



The distance that each particle moves from the position 
of rest is called the amplitude, xy. 

When a particle has moved from x to y, to t, to x, it is 
said to have made one complete vibration. 

The time required to make one complete vibration is 
called the period; and the number of vibrations the particle 
makes per second is called the frequency. 

75. Characteristics of Longitudinal Waves. — In longi- 
tudinal waves we have very nearly the same characteristics 
as in transverse waves. 

Instead of having crests and troughs, we have conden- 
sations and rarefactions. Figure 53 shows the particles as 



Wave Length 



t X U 

k= • — *l 



| •••*••• | + 



+ 



d\ 



Figure 53. 



^ Wave Length — >| 

Characteristics of a Longitudinal Wave. 



they would appear in a rubber band if a wave were traveling 
in it. 

The parts a and b where the rubber particles are crowded 



HOW LONGITUDINAL WAVES TRAVEL 71 

together, are called condensations. The parts o and d 
where the particles are stretched apart, are called rarefactions. 

The wave length is the distance from one condensation 
to the next, or from one rarefaction to the next. 

Amplitude, vibration, period, and frequency mean the 
same as in transverse waves. 

76. How Transverse Waves Travel. — Most transverse 
waves travel in a substance which has tensile strength, 
that is, a substance which will resist a pull. The wave 
moves from one position to another in this way : 

Figure 54 shows a wave in a rope, with some of its parts 
numbered. As the wave travels along the rope, the particles 




,8 .9 ,10 .11 ,12 

Figure 54. — The Start of a Transverse Wave. 

move up and down ; or, as the particles move up and down, 
the wave travels along the rope. It is the motion of the 
particles that produces the wave motion. 

In the figure, #1 has been to the top of the swing and 
has come back to its present position. Since #2 is on the 
same rope, it is pulled along after #1. Also, #3 is pulled by 
#2 ; and so on. Thus we see that the different particles are 
affected in a series, one after the other, and not all at once. 

To state it as briefly as possible : the wave travels by one 
particle pulling the next one after it. 

77. How Longitudinal Waves Travel. — Longitudinal 
waves may travel in substances that have tensile strength, 
but they do not depend on the pulling effect to make them 
travel. Instead, they depend on the crowding effect. 



72 WAVE MOTION 

As an example, take the longitudinal wave in a spring. 
(Figure 55.) The particles of the spring are all crowded 
together at d and e, and are all spread out at a and c. 

Now, since there is nothing to keep the spring stretched 
at positions a and c, and compressed at d and e, the crowded 
portions d and e will expand and tend to compress the parts 
a and c. 

If this action should stop when the spring is everywhere 
stretched alike, the wave would stop ; but it is the same as 

da e c 

Figure 55. — How a Longitudinal Wave Travels. 

when you run fast and then try to stop suddenly. You go 
farther than you intended. The same is true of the parts 
of the spring. The crowded portions expand too much, 
causing an overstretched portion; and the part that was 
stretched before is compressed. In this way, the crowding 
effect is passed along, and the wave is said to travel. 

78. Velocity of Waves. — Waves travel at different 
speeds. If the rope is stretched tight, the waves will travel 
faster than if the rope is loose. 
/( They would travel more slowly if 
the rope were large and heavy. 
On the other hand, the frequency 

Figure 56. -Velocity = q{ the Nation does not affect the 
Frequency x Wave Length. 

speed of the wave, nor does the 

amplitude. If the frequency is high, the waves are short; 

but if the frequency is low, the waves are long. 

During one vibration the wave travels 1 wave length, 

L. (Figure 56.) During two vibrations the wave travels 2 

wave lengths, 2 L ; while during three vibrations it travels 3 



^7 

3L- 






VELOCITY OF WAVES 73 

wave lengths, 3 L. From this we see that in N vibrations 
the wave will travel NL. 

Xow, N is the number that usually stands for the fre- 
quency ; so NL is the distance the wave will travel in 1 
second. 

The distance an object travels in a second is called its 
velocity; so the velocity of a wave is the frequency times the 
wave length; or 

Velocity = Frequency X Wave Length 
or V = NL. 



CHAPTER VI 
SOUND 

79. Definition of Sound. — Sound may be defined as a 
vibration of such a frequency that it may be detected by the ear. 

There are three things necessary for sound : (1) some 
vibrating object to start the vibration; (2) some medium 
to carry the vibration ; (3) something to receive the sound. 

The vibrating object to start the vibration may be a tun- 
ing fork, piano wire, bell, drum, etc. 

The air is the medium which usually carries the waves 
from the vibrating body to the ear which receives it. Water 
will do this very well ; and, in fact, any material body will 
carry the vibration. A vacuum will not. This can be 
shown by placing an alarm clock in a jar and then exhausting 
the air with a pump. The clock will become inaudible, but 
when the air is let in again it can be heard. 

The thing that usually receives the sound is the ear, 
but sometimes the vibration is detected by other things. 

80. Nature of Sound. — Sound waves travel through the 
air, but we cannot see the effect, since the air is transparent. 
Suppose that the air were made so we could see it, and that, 
just as a sound wave passed through it, an instantaneous 
photograph were made of the air. How would it look ? 

Figure 57 shows the condition of the air at a certain in- 
stant when a sound wave is passing through it. At the point, 
a, where the vibration started, the air is compressed. Around 

74 



VELOCITY OF SOUND 



75 




Figure 57. — A Sound 
Wave in Air. 



this the air is rare, b ; still farther out, it is compressed, c ; 
and it is again rare at d, etc. 

If pressure gauges were placed around in different parts 
of the room while the sound-wave was passing, some would 
show high pressures while others showed low pressures. 
This is because the vibrations crowd the air together at some 
places and stretch it out at others. These places are in the 
shape of spheres. The spheres are 
alternately places of high and low 
pressures. 

We have described the air at an 
instant while the wave is traveling 
through it. The next question is, how 
do the waves travel ? 

Sound waves are longitudinal, and 
depend on the crowding effect for their 
motion. For example, in Figure 57, a, c, 
etc., are at high pressures ; while b, d, etc., are at low pres- 
sures; so the air in the high pressures will push outward, 
crowding the air in the low pressures. This causes the air 
at the low pressures to become condensed, and form high 
pressures. In this way the high pressures travel outward. 
The low pressures follow in alternate order. 

You will notice that each particle of air moves only back- 
ward and forward, while the wave always moves forward. 

81. Velocity of Sound. — At 0° C. sound travels 1087 
feet per second. Examples are common which show that 
sound waves take time to travel. You can always see the 
steam before you can hear the whistle. Often you can see 
a carpenter hit a nail and later hear the sound. As in the 
case of all waves, V =NL. This formula is used in find- 
ing the velocity of sound. 



76 SOUND 

82. Effect of Temperature on Velocity of Sound. — You 

will notice that the temperature 0° C. was mentioned when 
the velocity of sound was given as 1087 feet per second. 
This is because a rise or fall in temperature changes the 
velocity of sound. A rise of 1° C. makes the velocity 2 
feet per second greater; and a fall of 1° C. makes the velocity 
2 feet per second less. 

Thus at 20° C. the velocity will be 1087 + (2 X 20) = 1087 + 40 
= 1127 feet per second. 

Since a rise in temperature causes air to expand, at a 
higher temperature the air is less dense, and is thus more 
easily moved. This explains the change in velocity with a 
change in temperature. 

83. Natural Free Period. — Any object such as a pendu- 
lum, a tuning fork, a swing, a string, etc. will vibrate with a 
certain period if allowed to swing freely. This period is 
called its natural free period. 

84. Resonance. — In starting to swing some one, the 
push must always come at a certain time. The push must 
be in unison with the motion of the swing. This is called 
resonance. 

Bridges can be set in motion if the even step of those 
crossing the bridge coincides with the natural free period 
of the bridge. For this reason, soldiers break step while 
crossing bridges. 

One tuning fork will be set in vibration by another, if 
they have the same natural free period. This is true of all 
musical instruments. 

The principle of resonance can be stated in this manner : 
Any object free to vibrate will be set in motion by periodic dis- 
turbances coming in the natural free period of the object. 




HOW WE HEAR 77 

85. The Ear. — The ear is the instrument with which 
we receive sound. The receiving is done in accordance 
with the principle of resonance. Figure 58 shows a section 
of the ear. The part (a) is that which we can see outside the 
head, and is called the external ear. From this a tube leads 
into the middle ear (b). Over the end of this tube is stretched 
a membrane (d) called the 

ear-drum. In the middle ear, 
attached to the ear-drum, is 
a series of three little bones. 
The last of these fits up 

against the end of a spiral 

, nil 77 • Figure 58. — Diagram of the 

tube called the cochlea or inner Ear 

ear (c). 

The cochlea is a bony tube making two and one half turns 

like a snail shell. This tube is filled with a liquid; and 

stretched from one side to the other are about 7000 strings, 

all of different lengths, and ranging in frequency from about 

16 to 10,000 vibrations per second. The tube (e) is the 

eustachian tube, which leads from the middle ear down into 

the throat. 

86. How We Hear. — A sound wave consists of a con- 
densation and a rarefaction, or a high and a low pressure. 
The external ear acts as. a funnel and directs the sound wave 
into the tube to the ear-drum. When the high pressure 
strikes the ear-drum, the membrane is pushed inward, and 
then when the low pressure comes it is pushed outward. 
This sets the three bones in motion, and the small 
stirrup-shaped bone hammers on the opening to the 
inner ear. This makes the liquid in the shell-like tube 
vibrate the same as the air outside the ear. One of the 
7000 strings — the one that has the same natural free 



78 SOUND 

period — will be set to vibrating by the principle of reso- 
nance. 

Thus far the process is purely mechanica 1 , and would 
take place whether the person were awake, asleep, or even 
dead. 

To distinguish between different sounds, or even to become 
conscious of them, is a psychological process. Each of the 
7000 strings has a nerve to the brain. Here it affects its 
own particular brain cell, thus making the person conscious 
of a sound. After many similar experiences the person is 
able to recognize a sound and distinguish it from other 
sounds. 

If sounds of different frequencies come into the ear, the 
several corresponding strings will vibrate, and the person 
hears a combination of sounds. 

87. Reenforcement, Interference, and Beats. — If two 
sound waves travel out together and are of different fre- 




Figure 59. — Reenforcement, Interference, and Beats Illustrated. 

quencies, they will reenforce one another at times, and 
interfere with one another at other times. 

Figure 59 shows two waves of different frequencies start- 
ing out together. At a they are making condensations and 
rarefactions at the same time, and thus they increase the 
effect, or reenforce one another. At b one wave has vibrated 
faster than the other, and is making a rarefaction while 



PITCH 79 

the other is making a condensation. This is an attempt 
to make both a high pressure and a low pressure at the 
same place at the same time. The result is neither. One 
interferes with the other. 

When the two waves reenforce one another, a loud sound 
is heard, and this is called a beat. A beat occurs every time 
one vibrating body gains one vibration on the other. 

If the frequencies of the vibrating bodies do not differ 
by more than ten, the ear is able to distinguish the separate 
beats ; but if they differ by more than ten, then the beats 
come so fast that the ear hears the series of beats as a new 
sound, and not as a series of separate sounds. 

88. Characteristics of Sound. — Sounds differ from one 
another in three different ways. These differences are 
called the characteristics of sound, and are named intensity, 
pitch, and quality. 

89. Intensity. — The intensity of sound means its loud- 
ness, and depends upon the amplitude of the vibration. A 
bell struck very hard with a hammer will give off a loud 
sound because the sides of the bell are made to swing with a 
large amplitude. As the amplitude gets smaller, the sound 
dies out and finally stops. 

90. Pitch. — The pitch depends upon the frequency of 
the vibration. A string vibrating 256 times per second has 
a different pitch from one vibrating 384 times per second, 
even if they are struck with the same force. On the other 
hand, a string may be struck gently or hard, and it will 
always give off the same pitch. So the pitch is independent 
of the amplitude. 

A pitch is said to be high or low, according to the frequency 
of vibration. The greater the frequency, the higher the 
pitch. 




80 SOUND 

91. Quality. — The quality of a sound depends upon its 
overtones. The overtone is the thing which makes it possible 
to distinguish one person's voice from another's, or to tell 
the difference between a piano and a violin, etc, 

92. Fundamental and Overtones. — When an object, 
such as a violin string, is giving its lowest tone, it is said to 
be giving its fundamental. The string vibrates back and 

forth as a whole, just like a rope 

that is being swung for some one to' 

jump it. We are apt to think this 

Figure 60. — Vibration of a is the only way a string will vibrate, 
Str.ng m Segments and fc ^ ■ .^ The ^ 

also as a Whole. t _ & 

will break up into segments which 
vibrate and in that way give off a higher tone. These 
higher tones are called overtones. 

A string may be giving several overtones and the funda- 
mental at the same time. It is the presence of the over- 
tones that changes the quality of the sound. 

Figure 60 shows a string vibrating as a whole and also in 
segments. 

93. Analysis of Sound Waves. — It has been known for 
many years that sound waves consist of fundamentals and 
overtones, but it is hard to tell just what overtones are 
present. In other words, it is hard to analyze a sound wave 
and tell just what waves it is made of. 

During the latter part of the nineteenth century a scientist 
named Helmholtz succeeded in analyzing sound waves. 
He made hundreds of resonators (Figure 61), all of 
different sizes, ranging from about a half-inch in diameter 
to several feet in diameter. By testing a certain sound 
with each of these hundreds of resonators he was able to 
determine which ones were in tune with that sound. The 



LAWS OF VIBRATING STRINGS 81 

ones that had the same free period vibrated; the others 
did not. 

The most recent and most successful attempt to analyze 
sound waves was made by Dr. Dayton Miller of Case 
School of Applied Science, who is still working on the prob- 
lem. He has made a machine which will transform the 
sound waves into a vibrating ray of light, so that the wave 
can be thrown upon a screen and seen with the eye. He also 
throws this ray on a photographic plate and takes a picture 
of the wave, making it possible to study the wave at leisure. 






a be 

Figure 61. — Helmholtz Resonators. 

Dr. Miller is now perfecting another machine, which will 
analyze the wave after it has been taken on a photographic 
plate. When this is successfully accomplished, he will 
be able to take any sound wave and tell how many and what 
overtones are present. 

With Dr. Miller's machine the differences between singing 
voices are easily seen. Some singers have many harmonious 
overtones, while others have very few. 

Figures 62, 63, 64, and 65 show samples of waves given 
by different singers. 

94. Laws of Vibrating Strings. — The pitch of a string 
may be changed in three ways : by changing (1) its length, 
or (2) its tension, or (3) its diameter. The tighter it is, the 



82 



SOUND 




Figure 62. 



Photograph of Sound Wave Produced by Speaking 
the Vowel "a" as in "Father." 




Figure 63. — Photograph of Sound Wave Produced by the Soprano 
Singing Alone in the Sextet from "Lucia." 




Figure 64. — Photograph of Sound Wave Produced by the Soprano 
and Baritone Singing Together in the Sextet from " LucrA." 



RESONANCE IN CLOSED PIPES 



83 




Figure 65. — Photograph of Sound Wave Produced by All Six 
Singing Together in the Sextet from " Lucia." 



faster it vibrates; the longer it is or the thicker it is, the 
slower are its vibrations. 

The laws concerning these three things are stated as 
follows : 

(1) The diameter and tension remaining the same, the 
frequency of a string varies inversely as its length. 

(2) The length and tension remaining the same, the fre- 
quency of a string varies inversely as the diameter. 

(3) The length and diameter remaining the same, the 
frequency of a string varies directly as the square root of the 
tension. 

95. Resonance in Closed Pipes. — If a tuning fork is 
struck and then held over a pipe closed at the bottom, the 
pipe will reenforce the sound of the at 
fork, provided that the tube is of the \^ 
proper length. 

When the fork moves from a to b 
(Figure 66), a condensation is made 
in front of the fork and travels down 
the tube to the bottom and back to 



the mouth again, while the fork moves Figure 66 . _ Resonance 
down to b. At this instant the fork in a Closed Pipe. 



84 I.. SOUND 

starts back toward a, forming another condensation in 
front of the fork ; but since a condensation is already com- 
ing out of the tube at this instant, this forms a double con- 
densation, making a loud sound. 

In the same way the rarefactions are reenforced. This 
action will take place only when the tube is of the proper 
length. The reflected condensation must be just coming 
out of the tube when the fork is ready to flip back from b to 
a ; and the reflected rarefaction must be coming out when 
the fork is ready to flip back from a to b. 

Now, since a condensation travels down and back, or 
twice the length of the tube, while the fork goes from a to 
b, or one half vibration, the sound will travel four times the 
length of the tube during a whole vibration. Therefore the 
closed pipe is one fourth wave length. 

By this method the velocity of sound may be determined. 
The wave length is found by multiplying the length of the 
tube by four. The frequency is al- 
ways marked on the fork. Then, by 
formula : 

V = NL. 

96. Resonance in Open Pipes. — 

If the pipe is open instead of closed 
at the bottom (Figure 67), the con- 
densation will travel down to the end, 
Figure 67. - Resonance and m then t & rare f ad i on in _ 

in an Open Pipe. m J 

stead of a condensation; so the fork 
must be back at a again before this rarefaction gets to the 
top. That is, while the sound travels down and back, the 
fork must make a complete vibration. Therefore the pipe 
is one half wave length. 



CHAPTER VII 
BASIS FOR MUSIC 

97. Music and Noise. — The prime difference between 
music and noise is that in music the sounds have rhythm 
while in noise they do not. By rhythm is meant that the 
sounds come at regular periodic intervals. 

The music of the savages consists almost entirely of beating 
time, while the music of civilized people goes farther than 
this, and consists of rhythm and harmony. 

98. Harmony. — Two or more tones are said to be in 
harmony if their combination is pleasant to hear. Har- 
mony, then, is the combining of musical tones, according to 
given laios, so that they will be pleasing to the ear. 

One of the laws of harmony is, the ratios of two tones must 
be in a simple ratio if they are to be in harmony. By " simple 
ratios " is meant, such ratios as {, t, i, i, i, i, r, i> etc. 

The reason why tones having their frequencies in simple 
ratios are harmonious is a matter of supposition. It is 
supposed that the mind likes system, and, more than that, 
simplicity of system. The most simple method in which 
soldiers can march is in step; the next simplest is every 
other soldier making two steps to his neighbor's one; the 
next is three steps to two; and so on. As soon as the ratio 
gets into large numbers, the mind fails to grasp the system, 
and the marching soldiers become a mob. 

The same is true of sound. When the ratios are simple, 

85 



86 BASIS FOR MUSIC 

the mind grasps the relation and is pleased ; but when the 
ratios become complex, the mind fails to detect any relation 
whatever, and a discord results. 

99. Major Triads. — When the frequencies of three tones 
are in the ratio 4:5:6, those three tones are called a triad. 
In music there are three triads, called major triads. They 

are : 

1. Tonic - C, E,G 

2. Dominant — G, B, d 2 

3. Subdominant — F, A, ci 

100. Major Scale. — The eight notes which form the 
major triads, when arranged in the proper order, form what 
is called the major scale. 

CDEFGABc 2 

The frequency of each of the tones in the major scale 
can be found by the ratios of the major triads. 



4:5:6 



The frequency of C can be taken as any number, and then 
the frequencies of each of the others can be determined from 
it. 

If 




C =256 




C = 256 


E 5 

C ~l 




G 6 
C 4 


E = lc 




.-!•< 


E = \ ■ 256 = 
4 


= 320 


-i- 



256 = 384 



By this method the frequencies of all notes can be found. 
Construct the major scale and calculate all the frequencies. 



THE CHROMATIC SCALE 87 

101. The Musical Interval. — The ratio of the frequencies 
of any two tones is called the musical interval between those 
tones. 

The musical intervals between consecutive tones in the 
octave, and the intervals between each tone and C are given 
as follows : 

Letter C D E F G A B c 2 

Frequency .... 256 288 320 341| 384 426f 480 512 
Interval between con- 

coon+i^ tr^Ti^c 9 10 16 9 10 9 16 

secutive tones . . & ~9' is s ~w~ 8" T5" 

Interval between each 

tone and C ... 1 f f If I. ¥ 2 

There are a few musical intervals of more importance 
than others, and these are given special names. Thus f = 
unison; f = a fifth; i = a fourth; f = a major third; ff = 
a half step; and f = an octave. 

102. The Chromatic Scale. — For certain purposes it is 
often advisable to start triads on other notes than C, G, and 
F. This requires other notes than those in the major scale. 
By starting triads on each of the other notes of the major 
scale we have : 

D : Zi : X 2 



A X i: Z 2 [^ 4:5:6 
B:Zi:Z 2 J 

Figuring out the frequencies of these unknown notes, we 
find they are in the first triad : 

f -\i *-| -288 = 360 
J- a = |; Xi=\ ■ 288=432 



88 BASIS FOR MUSIC 

Now, 360 falls between F and G, and 432 falls between A 
and B; so they are called F -sharp and A-sharp, respectively. 
Thus the first triad is D, F -sharp, and A-sharp. 

When all these unknown frequencies are calculated, it 
is found that there are five new notes which fall in between 
the other notes of the major scale, and a new scale is built 
up, using the major scale, with the five new notes added 
in their proper places. This new scale is called the chromatic 
scale. 

The notes in it are : 

C, C-sharp, D, D-sharp, E, F, F-sharp, G, G-sharp, A, A-sharp, B, c 2 J 

103. Tempered Scale. — The 'musical intervals between 
the consecutive notes in the chromatic scale are not all 
equal. But in the piano and similar instruments the notes 
are wade at equal intervals. This new scale is called the 
tempered scale. The musical interval between consecutive 
notes is 

^2 = 1.095 

This musical interval is calculated by this method : 
There are tivelve equal intervals in the tempered scale. 
Suppose we let x equal the numerical value of this interval. 

Then C-sharp = C ■ x 

D = C-sharp • x = C ■ x • x 
D-sharp = Dx = Cxxx. 

And so on for the complete scale. 

Therefore c 2 = C • x 12 ; 
but c 2 = C ■ 2. 

Therefore z 12 = 2, _ 
or x = y'2.. 

104. Standard Pitch. — In order that a piece of music 
may be played as intended, there must be a standard pitch 



MUSICAL INSTRUMENTS 89 

for C. There are several standards, the commonest being 
the " International Standard Pitch," which makes C = 261. 
105. Musical Instruments. — The student is here asked 
to report on one musical instrument, covering the following 
points : 

1 . Description of the instrument. 

2. How the sound is produced. 

3. How the pitch is determined. 

4. What the principal use of the instrument is. 



Review Problems 

1. Give five examples of wave motion. 

2. Distinguish between transverse and longitudinal waves. 

3. What are the characteristics of transverse waves? Define each. 

4. What are the characteristics of longitudinal waves? Define 
each. 

5. Explain how transverse waves travel. 

6. Explain how longitudinal waves travel. 

7. If a rope be shaken at the rate of 3 vibrations per second, and 
the waves are 10 feet long, how fast do the waves travel? 

8. Explain the nature of sound. 

9. If 3 seconds after you see the lightning you hear the thunder, 
how far away was the lightning? Take the temperature as 18° C. 

10. Why does a vase, or any other small article in the room, often 
rattle when the piano is played ? 

11. Why is it dangerous for the audience to stamp the feet in a large 
auditorium ? 

12. Describs the ear. 

13. How do we hear? 

14. Why do heavy explosions, such as the firing of cannon, often 
cause deafness? 

15. What are beats ? 

16. What is the cause of " dead points " — places where it is 
difficult to hear — in an auditorium ? 



90 BASIS FOR MUSIC 

17. Name the characteristics of sound. Upon what does each 
depend ? 

18. What is the difference between a 'sweet" and a " harsh " 
voice ? 

19. If two strings are the same, except that one is 40 cm. long and 
the other is 60 cm. long, what is the ratio of their frequencies? If 
the 40-cm. string vibrates 300 times per second, what is the frequency 
of the other ? 

20. Why are some of the strings on a piano large and others small ? 

21. How does a piano tuner tune a piano? Why does this change 
the pitch ? 

22. Why does a pipe organ have many pipes, all of different lengths ? 

23. Explain how to find the velocity of sound. 

24. What is rhythm ? Harmony ? 

25. What is the reason why tones must be-in simple ratios to be in 
harmony ? 

26. Construct a major scale, using C as 96. 

27. Construct a chromatic scale, using E as 400. 

28. What is the tempered scale? 

29. Why is the common musical interval between consecutive notes 
in the tempered scale 1.059? 

30. Name two other standard pitches besides the International 
Standard Pitch. (Outside reference.) 

31. What is the use of the sounding board in a piano ? 

32. Why does a phonograph give a higher pitch when run fast? 

33. What changes the pitch of a slide trombone ? 

34. What changes the pitch of a cornet? 

35. Why does the piano have the tempered scale? Figure out the 
frequencies of all notes on the piano, using A as 435. 



CHAPTER VIII 
LIGHT 

106. Nature of Light. — Nobody knows the exact nature 
of light. Many theories have been offered, but none has 
been accepted as final. But we know a great deal about 
light, even if we do not know just what it is. In this dis- 
cussion, we shall take up facts already proved and mention 
some of the latest theories. 

It is definitely known that light is one of the many forms of 
energy, and that it has much in common with radiant heat. 

107. Theory of Production of Light. — In almost all 
cases, light is produced by something hot. {Fluorescence 
and phosphorescence are exceptions.) Our common sources 
of light are the sun, a fire, a candle, a lamp, or some other 
very hot body. 

It is thought that the rapid vibration of the molecules of 
the heated body sets up waves in the ether, and that the 
ether transmits these waves through space. These waves 
are of different lengths, depending upon the frequency of 
the vibration of the molecules. Those waves which are of 
the right length to affect the eye are called light. 

When a piece of iron becomes hot enough, it gets luminous; 
in other words, it gives off light. The molecules of the iron 
vibrate very rapidly, and this vibration sets up waves in 
the ether, which are transmitted in all directions. These 
waves we call light. 

91 



92 



LIGHT 



108. Propagation of Light Waves. — Just how the ether 
transmits these waves is still a mystery, but it is known that 
they are transverse, and that they travel in straight lines. 

109. Velocity of Light. — It is easy to find examples 
showing that sound takes time to travel, but all ordinary 
examples fail to show that the same is true of light, and 
for many centuries the transmission of light was thought 
to be instantaneous. 

110. Roemer's Method of Finding Velocity of Light. — 
The first man to prove that the passage of light requires 
time was Roemer, and he did it by accident. 





Figure 68. 



Relative Positions of Sun, Earth, Jupiter, 
and Moon of Jupiter. 



Roemer was an astronomer who lived during the seven- 
teenth century. About 1676 he was studying the eclipses 
of one of the moons of Jupiter by Jupiter. He found that 
the eclipses did not occur at regular intervals, as was ex- 
pected, but that for six months the time between eclipses be- 
came shorter and shorter, and then for the next six months 
it became longer and longer. (Figure 68 shows the relative 
position of the heavenly bodies under consideration.) 

Every time the moon of Jupiter came into the shadow 
of Jupiter, there was an eclipse. Roemer knew how long 



COMPARATIVE VALUE OF VELOCITY OF LIGHT 93 

it took the moon to make a complete revolution about 
Jupiter, and so assumed that eclipses ought to come at that 
interval. He made a schedule something like the follow- 
ing (assuming that it takes exactly 30 days for the moon to 
make a revolution) : 

1st eclipse 12 o'clock Jan. 1 

2d eclipse 12 o'clock Jan. 31 

3d eclipse 12 o'clock Mar. 1 

4th eclipse 12 o'clock Mar. 31 

oth eclipse 12 o'clock Apr. 30 

6th eclipse . 12 o'clock May 30 

7th eclipse 12 o'clock June 29 

8th eclipse 12 o'clock July 29 

9th eclipse 12 o'clock Aug. 28 

10th eclipse 12 o'clock Sept. 27 

11th eclipse 12 o'clock Oct. 27 

12th eclipse 12 o'clock Nov. 26 

13th eclipse 12 o'clock Dec. 26 

The earth being at E, at tha time of the first eclipse, 
Roemer found that at each occurrence the eclipses were 
behind the schedule more and more, and that six months 
later, when the earth was at E 2 , the eclipse occurred 1000 
seconds later than the scheduled time (12 o'clock, June 29). 
Then, for the next six months, the eclipses began to catch 
up with the schedule, and were exactly on time (12 o'clock 
Dec. 26) when the earth got back to E x . 

Roemer then reasoned that it took the light 1000 seconds 
to cross the earth's orbit, a distance of 186,000,000 miles. 

This gave the velocity of light as irjrjn = 186,000 miles 

per second. 

111. Comparative Value of Velocity of Light. — The 

velocity of light, 186,000 miles per second, is so great that 
the mind cannot appreciate it without some comparative 



94 LIGHT 

values. It means that a ray of light would travel nearly 
1\ times around the earth in one second. It would take a 
train, going at 60 miles an hour, over 4 months to travel 
as far as a ray of light can travel in one second. 

112. Shadows. — Since light travels in straight lines and 
will not go through opaque objects, it is easily shut off by 
putting one of these objects in its path. When light is 
shut off from a certain space by an object placed in the 
path of the light, that space is called a shadmv. A shadoio 
is the space from which the light has been cut off. 

A man walking on the sidewalk on a sunny day casts a 
shadow. Hold your hand in front of a lamp and your hand 
casts a shadow. The earth shuts off part of the sun's rays 
and casts a shadow. 

The shadow in each of these cases is the space back of 
the object. It is not, as we often think, the dark portion 

of the sidewalk or of the 
wall. Those are only cross 
sections of the shadows. 

113. Shadow from a Point 
Source of Light. — Figure 

Figure 69. -Shadow from a Point 69 shoWS a shadoW Cast b * V 
Source of Light. an object in front of light 

coming from a point source. 

The light travels out in all directions from point P, but 
that which strikes the rectangle abed is shut off, thus making 
the space S without light, or a shadow. The shadow, then, 
is a pyramid with the top cut off. 

Had the object been circular, the shadow would have 
been a cone with the top cut off. 

114. Shadow from a Large Source. — Most of our light 
comes from large sources and not from points. Figure 70 




SHADOW FROM A LARGE SOURCE 



95 



shows the shadow cast by an object (0) with a large source 
of light (S). 

It will be seen that the space above be and below ad is 
lighted by all of S. The space between ac and bd beyond 
the object gets no light at all, and so is totally dark. This 
is called the umbra (U). The space outside the umbra, 




Figure 70. — Shadow from a Large Source of Light. 



and still inside ad and be, is called the penumbra (P, P). 
This space is totally dark at ac and bd, but becomes lighter 
and lighter, as you go outward. That is, point y has more 
light than point x, because more of S is shining on it. 

Shadows play a great part in the arts both of painting 
and of sculpture. They also enter into the problems of 
proper illumination, and so will be further discussed under 
that topic. 



CHAPTER IX 



REFLECTION AND MIRRORS 



115. Reflection. — If a ray of light strikes a bright sur- 
face, it will be partially reflected. Reflection is the returning 
cf a ray of light into the same medium from which it came, 
ivhen it strikes another medium. 

One of the most common cases of reflection is seen when 
a ray of light strikes a mirror. Figure 71 shows a ray of 
ia light striking a mirror and being 
reflected. 

IR is the incident ray. RR is 
the reflected ray. MM is the 
mirror, and OP is the perpen- 
dicular to the mirror at the point 
where the ray IR strikes the 
mirror. 

The angle behveen the incident 
ray and the perpendicular to the 
mirror is called the angle of in- 
cidence. 

The angle between the reflected ray and the perpendicular to 
the mirror is called the angle of reflection. 

Light is ahvays reflected so that the angle of reflection equals 
the angle of incidence. This is called the Law of Reflection. 

116. Pencil of Rays. — So far we have spoken of rays of 
light. Light never goes in single rays, but in bunches of 

96 




M 

Figure 71. — Showing Reflec- 
tion of a Ray of Light. 



IMAGE IX A PLANE MIRROR 



97 




Figure 72. — A Pencil of Rays. 

The object is ab ; MM, the 

An image is the space occupied 



rays. A small bunch of rays is called a pencil of rays, and 

this is what we have to consider instead of single rays. A 

person gets a pencil of 

rays, or many pencils of 

rays, in his eye, instead 

of just single rays. 

(Figure 72.) 
117. Image in a Plane 

Mirror. — Figure 73 shows 

the image in a plane mirror. 

mirror ; and a'b' , the image. 

by ivhat is apparently the object itself. 

Rays are sent off in all directions from each point of the 

object. Let us consider the two points a and b, the head 

and tail of the object. There is just one pencil of rays from 
j.j each of these points 

which goes out, strikes 
the mirror at the right 
angle, and is reflected 
into the eye. 

The pencil of rays 
coming from a, after 
being reflected at c, 
appears to come from 
point a' ; and the 
pencil of rays coming 
from b, after being 
reflected at d, appears 
to come from b' '. 
By geometry it is easily proved that the image is as far 

back of the mirror as the object is in front, and on a line with 

the object, perpendicular to the mirror. 




V 



Figure 73. 



-Construction of an Image in 
a Plane Mirror. 



98 REFLECTION AND MIRRORS 

There are two kinds of images, real and virtual. 

A real image is an image through which the rays of light 
actually pass. 

A virtual image is an image through which the rays of light 
apparently pass, but do not. 

It will be seen by these definitions that the image in a 
plane mirror is virtual. Why? 

118. Concave Mirrors. — A concave mirror is a mirror 
which curves, and has the hollow side towards the object. 

There are several kinds of concave 
mirrors. The two most common ones 
are the spherical mirror (Figure 74) and 
the parabolical mirror (Figure 75). 

Figure 74. — a Spheri- rpj^ S pherical mirror is a portion of 
the surface of a sphere, every point of 

which is equidistant from one point (c) called the center of 

curvature. 

The parabolical mirror is the portion of the surface of a 

paraboloid and is of the shape shown 

in Figure 75. The parabolical mirror is 

much better than the spherical because 

it gives a perfect image, while the other 

does not. FlGURE 75> " A Para ~ 

bolical Mirror. 

119. Meaning of Terms. — In Figure 

76 the point c is the center of curvature, and is equidistant 
from all points in the surface of a spherical mirror. The 

line XO is the prin- 
cipal axis, and is the 

+ x line passing through 

the center of curvature 

r- ~ „ (c) and the center of 

Figure 76. — The Principal Points of a v ' # 

Spherical Mirror. the mirror (0). 





IMAGE IN A CONCAVE MIRROR 



99 



The focus of a mirror is the point at which the image 
is located. The point / is the principal focus, and is 
the point at which all rays parallel to the principal 
axis are focused. The principal focus is located at one 
half the distance from c to 0. The focal length is the dis- 
tance (Of) from the center of the mirror to the principal 
focus. 

120. Image in a Concave Mirror. — Figure 77 shows the 
construction of an image in a concave mirror. 

First, draw ad from a, the head of the object, parallel 
to the principal axis. Since this is a ray parallel to the 

y 




Figure 77. — Construction of Image in a Concave Mirror. 



principal axis, it must be reflected through the principal 
focus /. This determines line dx. 

Second, draw ag from a through the center of curvature c. 
This ray is reflected back upon itself, since it strikes the 
mirror perpendicularly. The point a', where these two 
reflected rays meet, is the head of the image. 

Third, locate the tail of the image in the same way. This 
completes the construction of the image. 

This image is seen to be real, inverted, and smaller than the 
object. The image may be located by this method for any 
position of the object. The description of the image can 
then be easily given from the figure. 



100 REFLECTION AND MIRRORS 

121. Convex Mirrors. — A convex mirror is a curved 
mirror which has the hollow side of the curve away from the 
object. 

The same terms, focus, axis, etc., apply to the convex 
mirror as to the concave mirror. 

122. Image in a Convex Mirror. — The construction of 
the image in a convex mirror is the same as for the concave 
mirror. Draw the two lines from the head of the object, 




Figure 78. — Construction of Image in a Convex Mirror. 

one (ad, Figure 78) parallel to the principal axis, and the 
other (ac) through the center of curvature. When reflected, 
these two rays pass through the principal focus and back upon 
themselves, respectively. Where they meet {a') is the 
image of the head. The image of the tail (&') is located in 
a similar manner. 

In this case the image is virtual, erect, and smaller than the 
object. 

123. Applications of Mirrors. — I. Plane Mirror. The 
general use of the plane mirror as a looking glass is too 
familiar to need discussion. 

2. Concave Mirror. The most general use of the concave 
mirror is that of a reflector. Since all parallel rays come 
together at the principal focus, it is seen that the rays from 
a source of light placed at the principal focus will be sent 
out as parallel rays. (Figure 79.) 



APPLICATIONS OF MIRRORS 101 

The automobile headlight is an example of this. The 
bulb is so placed that the filament of the lamp is very near 
the principal focus of the reflector. This sends the rays 
out in nearly parallel beams. The correct position of the 
filament is just beyond the principal focus, but close to it. 
This makes the rays cross and then diverge slightly, so that 
a large area of the road can be seen. The same use is also 
made in many different kinds of lamps. 

The concave mirror is used in all telescopes of the re- 
flector type. The largest telescope of this sort has just 
been completed by the Warner Swazey Company, of Cleve- 
land, to be used at the Canadian observatory at Victoria. 




1-4- 



Figure 79. — A Concave Mirror used as a Reflector. 

The concave mirror for this telescope is 72 inches in diameter, 
and, like all high-grade concave mirrors, is of the parabolical 
shape. The telescope will be used to take photographs of 
distant stars. The mirror is large so that many rays of the 
star are focused at the image. 

3. Convex Mirror. The convex mirror is often used on 
automobiles to give the driver a view of vehicles behind 
him. It is usually placed on the front fender or attached 
to the side of the windshield. The mirror gives a small 
but clear image of everything in the rear. 

Large spheres with mirror surfaces are often placed in 
flower gardens to add to the decorations and to give beau- 
tiful images of the walks and flowerbeds. 



102 



REFLECTION AND MIRRORS 



Another use of the convex mirror is that of the " vanity- 
mirror " carried in ladies' hand bags or pocketbooks. It is 
much preferred to the plane mirror, for even a small one an 
inch in diameter will give an image of the whole face. 








Figure 80. — Peculiarly Shaped Mirrors. 



4. Peculiarly Shaped Mirrors. There are many peculiarly 
shaped mirrors, such as are found in " hilarity halls," etc. 
Figure 80 shows a few of these. Due to the peculiar shapes, 
the images are distorted and afford amusement for the 
patrons. 



CHAPTER X 



REFRACTION AND LENSES 



124. Refraction. — The term refraction is very often 
confused with the term reflection, but it must be borne in 
mind that the two mean entirely different things. 

Refraction is caused by the change in velocity of a ray of 
light when it passes from one medium to another. This causes 
a bending of the ray when it strikes at an 
angle other than 90°. 

If a lead pencil be put into a beaker of 
water (Figure 81), it looks as if the lead 
pencil were bent at the water line. If you 
try to touch an object under water very 
quickly, your hand will pass over the object, 
showing that the object appears higher than 
it really is. If you look through a poor grade 
of window glass at some straight line, such 
as the side of a tall chimney, the line looks 
jagged and crooked. (Figure 82.) 

All these illusions are caused by refraction. 

125. Refraction Explained. — Figure 83 shows a ray of 
light (A) passing from air, through a piece of plate glass, 
back into air. 

The small lines ab represent the wave front of the ray. 
A ray of light always travels at right angles to its wave 
front ; so the wave front determines its direction. 

103 




Figure 81. — A 
Pencil Looks 
Bent at the 
Surface of 
the Water. 



104 



REFRACTION AND LENSES 




The ray travels in a straight line until it strikes the 
glass. The side a strikes first, and so is retarded, since 

light cannot travel in 
glass as fast as in air. 
This allows b to swing 
ahead, since it is still in 
air. This continues until 
both a and b are inside the 
glass. Then they again 
go at equal speeds, giving 



Figure 82. -A Chimney Viewed through the ra y a straight path, 



Poor Window Glass. but one slightly deviated 

from its original path. 

At the other side of the glass, a comes to the surface 
first, and so swings ahead of b, for it now travels in air. 
It continues to do this until both a and b are again in air. 
Here they continue again at equal speeds, and the ray 
again goes in a straight line. 

If the two sides of the glass are parallel, the ray swings 
back just as much as it deviated in the first place. This 
makes its path parallel to its path before entering the glass, 
but not in the same line. 

If the two sides of the glass are not parallel, the ray 
will not be parallel with its first path, but will deviate ac- 
cording to the angle of the two surfaces. 

126. Meaning of Terms and Law of Refraction. — In 
refraction^ the incident ray is the ray before it strikes the 
refracting surface (AO for the first surface, and 00' for the 
second surface, Figure 83). 

The refracted ray is the ray after it strikes the refracting 
surface {00' for the first surface, and O'A' for the second 
surface) . 



INDEX OF REFRACTION 



105 



The angle of incidence is the angle between the incident 
ray and the perpendicular to the surface (angle i for the 
first surface, and angle i' for the second surface). 

The angle of refraction is the angle between the refracted 
ray and the perpendicular to the surface (angle r for tho 
first surface, and angle r' for the second surface). 




Figure 83. — Diagram Explaining Refraction of Light. 



The law of refraction : A ray of light passing from a rare 
medium into a denser medium always bends toward the per- 
pendicular, and a ray of light passing from a dense to a rarer 
medium always bends away from the perpendicular. 

127. Index of Refraction. — Different substances refract 
light in varying degrees. In order to compare and express 
these amounts of refraction a term called index of refraction 
is used. 



106 



REFRACTION AND LENSES 



The index of refraction is equal to the velocity in the rare 
medium divided by the velocity in the dense medium. 

Vrare 



Index of Refraction 



Vde 



There are two kinds of indexes of refraction, relative and 
absolute. 

The relative index of refraction is the index when the ray 
passes from one substance to another, and is correct for those 
two substances only. 

The absolute index of refraction is the index when the ray 
passes from a vacuum into a substance, and applies to that 
one substance only. 

The index of refraction is used principally in the manu- 
facture of lenses. The index determines the amount of 
curvature that the lens must have. It is the high index of 
refraction of the diamond that gives it its sparkle. 

128. Applications of Refrac- 
tion. — The applications of re- 
fraction are used in lenses and 
prisms. These will be discussed 
later. 

We have mentioned the effect 
of looking at a straight line 
through a poor grade of window 
glass. Explain this. 

It is a common thing to notice 
the wavy effect above a fire or stove (Figure 84). This is 
not heat waves, as so many think ; but it is due to refrac- 
tion. The air above the fire is heated and becomes less 
dense than the surrounding air. Light rays passing through 
these layers of air of unequal densities are refracted, giving 
the wavy effect. 




Figure 84. — Refraction of Light 
above a Hot Stove. 



CRITICAL ANGLE 



107 



Our atmosphere acts as a refracting substance to the 
sun's rays. For this reason we can actually see the sun 



( Sun j 
Evening 




Morning 



Figure 85. — Refraction of Light by the Earth's Atmosphere. 

before it is above the horizon in the morning, and also after 
it has gone below the horizon in the evening. (Figure 85.) 

129. Critical Angle.— 
Figure 86 shows what 
takes place when a ray 
of light passes from a 
dense medium, such as 
water, to a rare medium, 
such as air. 

A ray of light AO passes 
from the dense medium 
and goes into the rare 
medium at 0. According 
to the law of refraction, 
the ray is bent away from 
the perpendicular PP f , 
making the angle of re- 
fraction r larger than the angle of incidence i. (Figure 86.) 

Xow, if the angle of incidence is made larger and larger, 
the angle of refraction will become larger and larger also 




Figure 86. — Diagram Explaining 
Critical Angle. 



108 



REFRACTION AND LENSES 



and will always be greater than its corresponding angle of 
incidence. 

If the angle of incidence becomes large enough (i 2 ), the 
angle of refraction (r 2 ) becomes equal to 90°, and the re- 
fracted ray passes out along the surface of the dense medium 
{OB'). The angle of incidence is then called the critical 
angle. 

The critical angle is an angle of incidence which corresponds 
to an angle of refraction of 90°. 

130. Total Reflection. — Angle i (Figure 87) is the 
critical angle, and so the refracted ray OA' passes out 

along the surface of the 
dense medium, making 
the angle of refraction (r) 
equal to 90°. 

Now, if the angle of 
incidence is made still 
larger, such as i 2 , the 
angle of refraction be- 
comes greater than 90°. 
This makes the refracted 
ray return into the same 
medium in which it en- 
tered. But this is reflec- 
tion instead of refraction, 
and so the ray must obey the law of reflection, making the 
angle of reflection (r 2 ) equal to the angle of incidence (^). 

This is called total reflection, because none of the rays 
can be refracted, but all are reflected. 

Total reflection is reflection against a surface of a rare 
medium when the angle of incidence is greater than the critical 
angle. 




Figure 87. — Diagram Explaining 
Total Reflection. 



PRISMATIC WINDOW GLASS 



109 



< 






=> 


■ pf^ 


\. ' 




C\p * 


' P K 


^®^ 


\ 7^ * 


« 


> 



Figure 



38. — Positions of Prisms in a 
Lighthouse Reflector. 



It must be noted that total reflection takes place only 
when the ray is passing from a dense to a rare medium. 

APPLICATIONS OF TOTAL REFLECTION 

131. The Lighthouse Reflector. — The lighthouse re- 
flector is an application of total reflection. The source of 
light, a gas flame p 
or an electric light 
bulb, is placed at 
the center. (i, 
Figure 88.) Circular 
right-angled prisms 
(Figure 89) are 
placed around the 
light at P, P, P, etc. 
(Figure 88), forming 
an inclosed sphere. Instead of the prisms being far apart, 
as in the figure, they are placed so close together that no 
light gets out between them. 

The light coming from the center 
strikes one leg of the right-angled 
prism, enters the glass, and then 
strikes the hypotenuse at an angle 
greater than the critical angle. Total 
reflection takes place, and all the light is sent out in a 
parallel beam. By this means all the light is utilized, top, 
bottom, and sides. 

132. Prismatic Window Glass. — Very often it is im- 
possible by means of the ordinary windows to get sunlight 
into rooms shaded by other buildings, especially in large 
cities where " skyscrapers " are the rule. Prismatic window 
glass helps to do away with this difficulty. The light com- 




Figure 89. — A Light- 
house Reflector Prism 



110 



REFRACTION AND LENSES 



ing almost straight down (Figure 90) strikes the prismatic 

glass and is totally reflected into the room; 

133. Field Binocu- 
lars. — In the field 
binoculars, such as 
are used by officers 
of the army and 
navy, the light must 
pass a distance of 
several inches after 
it enters the instru- 
ment before it reaches 
the eye. To keep 
the instrument from 
becoming too long, 
the rays of light are 
reflected back and 
forth from one end 
to the other by 
means of right- 
angled prisms. 
Figure 91 shows a diagram of the path of a light ray in 

one tube of the binocular. 

134. A Fish's View of the Outside World. — It is rather 

interesting to note just 

how the outside world 

looks to the fish below 

the surface of the 

water. Figure 92 is a 

diagram showing how 

the rays of light come to the fish's eye. 

The sky and all objects above the horizon are seen through 




Figure 90. 



Use of Prismatic Window 
Glass. 




Figure 91. 



— Total Reflection Used in 
the Binoculars. 



LENSES 



111 



a cone whose angle is about 97°. Outside of this cone the 
fish gets rays coming from the bottom and reflected at the 
surface of the water. This makes the sky look as if it had 
a fringe of stones or 
grass, according to 
whether the bottom 
is stony or grassy. 

135. The Diamond. 
— As mentioned be- 
fore, the large index 
of refraction in a dia- 
mond gives it its 

sparkle. As the diamond has a large index of refraction 
and is cut with many facets, the light is reflected many 
times within the stone, so that there is scarcely an angle 
at which you can view it without getting a flash of light. 




Figure 92. — A Fish's View of the 
Outside World. 



LENSES 

136. Lenses. — A lens is a transparent body of such a 
shape that it will focus rays of light. There are two general 

classes of lenses : (a) con- 
verging, (b) diverging. 

A converging lens is a 
lens ivhich tends to bring 
the rays together after they 
pass through. (Figure 
93.) 

A diverging lens is a 
lens ivhich tends to send the rays farther apart after they go 
through. (Figure 94.) 

Lenses are of different shapes and are given specific 
names according to these shapes. (Figure 95.) In general, 




Figure 93.— Light Passing through a 
Converging Lens. 



112 



REFRACTION AND LENSES 



lenses that are thicker at the center than at the edges are 
converging, while those thinner at the center than at the 
edges are diverging. 

137. Meaning of Terms. — The 
line drawn through the centers of 
curvature of the two surfaces is 
called the principal axis. (CC, 
Figure 96.) 

The optical center (0) is the point 
on the principal axis, midway be- 
tween the surfaces of the lens. 

The principal focus (F) is the 
point at which all rays parallel to 
the principal axis are focused. 

The focal length (OF) is the distance from the optical 
center to the principal focus. 

The image is a point, or a series of points, at which the 
rays coming from an object are focused. The rays coming 




Figure 94. — Light Pass- 
ing THROUGH A DIVERGING 

Lens. 





bed c 

Figure 95. — Different Shaped Lenses. 



from one point of the object are focused at one point in the 
image. 

138. Image through a Converging Lens. — There are 
five possible settings for a converging lens : 

I. The object beyond 2 F. 



IMAGE THROUGH A CONVERGING LENS 



113 



To construct the image for this position (Figure 97) 
draw the lens and the principal axis ; locate the optical 
center and the principal focus. (Note: Every lens has its 
own focal length ; and if this is given, the principal focus 
can be located by it; but if the focal length is not given, 
then a focal length 
must be assumed.) 
Next, mark off F 
and 2 F on both sides 
of the lens, and place 
the object beyond 2 F. 

Now, there are an infinite number of rays passing from 
every point of the object, but two rays are sufficient to 
locate the image of any one point. Select the two rays, 
one of which is parallel to the principal axis, and the other 
which passes through the optical center. 

To locate the head of the image draw these two rays, 
the one parallel to the axis passes through the principal 




Figure 96. — Principal Points of a Lens. 




Object 



Figure 97. — Construction of Image when Object is beyond 2 F. 



focus F (because all parallel rays are focused at this point), 
and the one through the optical center passes on through 
the lens in a straight line (it really zigzags just a little at 
the lens). The point at which these two rays meet is the 
image of the head. 



114 



REFRACTION AND LENSES 



In the same way the tail of the image is located, thus 
locating the whole image. 

The description of an image gives four things : (1) posi- 
tion, (2) size, (3) whether it is erect or inverted, (4) whether 
it is real or virtual. 

When the object is beyond 2 F, the image is (1) between F 
and 2 F, (2) smaller than the object, (3) inverted, and (4) real. 

II. The object at 2 F. 

To construct the image with the object in this position, 
proceed exactly as in the former case. (Figure 98.) 




Object 



Figure 98. — Construction of Image when Object is at 2 F. 



The image is (1) at 2 F, (2) of the same size as the object, 
(3) inverted, and (4) real. 

III. The object between F and 2 F. 

Construct as before. (Figure 99.) The image is (1) be- 
yond 2 F, (2) larger than the object, (3) inverted, and (4) real. 




Object 



Figure 99. — Construction of Image when Object is between 
F and 2 F. 



IMAGE THROUGH A CONVERGING LENS 115 



IV. The object at F. 

Construct as before. (Figure 100.) The rays after passing 
through the lens are parallel, and so never meet. There- 
fore there is no image. 




Figure 100. — Construction of Image when Object is at F. 

V. The object between F and the lens. 

The construction is the same as before, except that the 
rays after passing through the mirror diverge, and so have 




Figure 101. — Construction of Image when Object is between F 
and the Lens. 

to be produced backward to determine the point where they 
meet. (Figure 101.) 



116 REFRACTION AND LENSES 

The image, then, is (1) on the same side of the lens as the 
object, (2) larger than the object, (3) erect, and (4) virtual. 

139. Image through a Diverging Lens. — There were 
five distinctive positions for the object in the case of the 
converging lens, but for the diverging lens there is only one. 
The image may be constructed in a similar manner to those 
already studied. 

Draw the two rays from each, the head and tail of the 
object. (Figure 102.) The two rays parallel to the principal 
axis diverge at such an angle that, if produced, they pass 




Object 



Figure 102. — Construction of Image through a Diverging Lens. 

through the principal focus. These produced rays meet 
the rays coming from the same points and form an image 
which is (1) between F and the lens, (2) smaller than the 
object, (3) erect, and (4) virtual. 

No matter where the object is, the image is formed as 
described above. If the object is a great distance away, 
the image approaches F ; and as the object comes closer to 
the lens, the image also comes closer to the lens, and gets 
larger. The image reaches the lens and becomes equal to 
the object when the object reaches the lens. 

APPLICATIONS OF LENSE3 

140. The Pinhole Camera. — The simplest camera that 
we have is illustrated by Figure 103. It consists of a light- 



THE LENS CAMERA 117 

tight box with a pinhole in the front. A sensitized plate 
or film may be placed at the back, and a picture can be 
taken. The principle of the pinhole camera is this : All 




Figure 103. — The Pinhole Camera. 



the rays allowed to pass through the pinhole from the same 
point of the object fall at the same point at the back of the 
box. A series of these points forms the image. 

141. The Lens Camera. — The pinhole camera is not 
satisfactory, for if the pinhole is very small, the image will 
be verv weak and dim ; and, on the other hand, if the hole 




Figure 104. — The Lens Camera. 

is made large, then the rays from the same point on the 
object fall over quite an area of the image, and this makes 
the image indistinct, or blurred. 

By the use of a converging lens, Fig. 104, the opening may 
be made large and, at the same time, the image may be 



118 REFRACTION AND LENSES 

kept sharp and distinct. This is an application of the con- 
verging lens with the object beyond 2 F. 

In order that all the rays coming through the lens from 
one point of the object be focused at a single point of the 



Figure 105. — The 40-Inch Telescope at the Yerkes Observatory, 
University of Chicago, Williams Bay, Wisconsin. 

This is the largest refracting telescope in existence. The tube is 
64 ft. long, 52 in. in diameter at the center, and the whole in- 
strument weighs 75 tons. 



THE EYE 119 

image, the lens must be ground with great care. This is 
why the best cameras are so expensive. 

The plate or film upon which the picture is taken is a 
piece of glass or other transparent substance covered with a 
gelatin. This gelatin is of such a composition that when 
sun light strikes it, it is made insoluble. When a picture is 
taken, the rays from the light parts of the object affect the 
plate more than the rays from the dark parts. Then, when 
the plate is " washed " the unaffected parts dissolve, leaving 
the insoluble part on the plate. 

The plate is then washed in a " fixing " solution, which 
makes the remaining gelatin hard to scratch. The plate is 
now called the negative. It has dark spots where the object 
is light, and light spots where the object is dark. 

For printing the pictures, either a paper or glass with a 
sensitive gelatin is used. The " negative " is laid over the 
sensitive paper or glass and held in the sun for a short time. 
The sensitive plate is affected just as the negative was 
when it was made, except that the dark and light spots 
are reversed, thus reproducing the object as it was seen. 

As all these processes must be done with painstaking 
care, photography is quite an art. 

142. The Eye. — The eye is also an application of the 
converging lens when the object is placed beyond 2 F. 

The human eye is about an inch in diameter and has 
three coats. The outer coat is very thick and strong, and 
is called the sclerotic coat. (Figure 106.) This sclerotic coat 
covers the entire eyeball, but at the front it is transparent 
and this portion has the name cornea (C). 

The next coat (D) is dark in color, and is called the choroid 
coat. At the front, the choroid coat forms a kind of curtain, 
called the iris (I). The iris is the part that gives color to 



120 



REFRACTION AND LENSES 




Figure 106. — 1 he Eye. 



the eye. At the back of the eye is a third coat (R) called 

the retina. This is nervous tissue composed of millions 

of small nerve cells. 
These cells are divided 
into three classes. In 
one class are those 
affected by red light; 
in another class are 
those affected by green 
light; and the third 
class is composed of 
those affected by blue 
light. These different 
kinds of cells are not 

in separate groups, but are scattered all over the retina, so 

that every point has all three kinds. 

At the front of the eye, fastened into the choroid coat, 

are muscles (m, m). 

These muscles are so attached that they stretch or relax 

a small membrane sack which contains the crystalline lens 

{C. L.). This crystalline lens is a transparent, jelly-like mass, 

and is a true lens. 

143. How We See. — When an object is held before the 
eye, an image is focused by the crystalline lens upon the 
retina. The nerve cells are affected according to the color 
of the light which falls on them. Impulses are sent to the 
brain, and we become conscious of the image. 

A further study of color will be taken up later, and the 
subject of the eye should then be reviewed. 

144. Defective Eyes. — There are many defects of the 
eye, but we will mention only three : short-sightedness 
(myopia), long-sightedness (hypermetropia), and astigmatism. 



DEFECTIVE EYES 



121 




Figure 107. — A Short-sighted Eye. 



Short-sightedness is caused by one, or both, of two things. 

The eyeball is too long, or the crystalline lens is too thick. 

When the image falls 

in front of the retina, 

the person has to 

bring the object very 

near the eye to get 

the image to move 

back upon the retina. 

(Figure 107.) 
To correct this defect, diverging lenses should be used 

for eye-glasses. This makes the image fall upon the retina 

when the object is 
held at the natural 
position. (Figure 

108.) 

Long-sightedness is 
just the opposite of 
short-sightedness, 
and is caused by 
just the opposite 

things. The eyeball is too short, or the lens is too thin. 

This makes the image fall back of the retina, so that it is 

necessary to hold the 

object far away in ^ y^^ ^*\ 

order to get the 

image to fall on the 

retina. (Figure x ^ ^ / 

109.) Figure 109. — A Long-sighted Eye. 

Glasses to correct 
;his defect should be converging lenses. (Figure 110.) 

Astigmatism is the most serious of the three defects, and 




Figure 108. — A Short-sighted Eye 
Corrected. 



t==4^ 



122 



REFRACTION AND LENSES 



is much the hardest to correct. It may be caused by several 
things, such as irregularities in the thickness or texture of 

the cornea, or in the 
crystalline lens. 

Figure 111 shows 
an eye with irregular 
thickness of the 
cornea. The defect 
must be corrected by 
having glasses ground 
to fit this one special 
Figure 112 shows an at- 




Figure 1 10. —A Long-sighted Eye 
Corrected. 




case, and this requires an expert, 
tempt to correct astigmatism. 

145. The Life-size Picture 
Camera. — This camera is 
just like the ordinary camera 
except that the box is very 
long and large and the lens 
has a greater focal length. 

This is an application of the second position of the con- 
verging lens. The object is placed at 2 F in front, and the 

plate is placed at 2 F, 
back of the lens in the 
box. (Figure 113.) 

It is used for taking 
photographs of machinery 
and parts of machinery, 
and sometimes of per- 
sons. 

146. The Projection Lantern. — The projection lantern 
(Figure 114) is an application of the converging lens with 
the object placed between F and 2 F. 




Figure 111. — An Astigmatized Eye. 



Figure 112. — An Astigmatized Eye 
Corrected. 



THE MOTION-PICTURE MACHINE 



123 



An arc light is used to illuminate the object (0, Figure 
114), which is usually a picture on a glass plate called a 
slide. In order that more of the light from the arc may 
strike the object, and in order that it may come in parallel 




Figure 113. — A Life-size Picture Camera. 

rays, condensing lenses (c, c) are placed between the arc 
and the object. 

Xow, the slide or object is placed between F and 2 F be- 
tween the light and the lens, and the image is thrown on a 
screen some distance in front, the image appearing very large 




Figure 114. — A Projection Lantern. 

and inverted. To make the image erect, the slide is placed 
in the machine upside down. 

147. The Motion-picture Machine. — The motion-picture 
machine is merely a projection lantern with an attachment 
for changing the slides at the rate of 16 or more per second. 

When images fall on the retina of the eye their effects 
tend to linger ; that is, after the image has left the retina the 



124 



REFRACTION AND LENSES 




Figure 115. — A 
Dark Lantern. 



nerves do not lose the effect immediately, and we continue 
to see the image for about -Jt of a second after it is gone. 

Now, by throwing pictures upon a screen at the rate of 
16 per second the last picture has not left our mind before 
the next one has come. This makes the 
pictures appear to be continuous. 

Thus we see the motion that takes 
place if pictures are taken at the rate 
of 16 per second and reproduced at that 
rate. 

The pictures are taken on a long film 
and are about ■§■" XI" in size, This film is run off a reel, 
through the motion-picture machine, on to another reel. 

148. The Dark 
Lantern. — A good 
example of the con- 
verging lens with the 
object at F is the 
dark lantern. (Figure 
115.) 

Here the light is 
placed at the prin- 
cipal focus, and after 
passing through the 
lens it goes in a 
parallel beam. 

149. The Magnify- 
ing Glass. — Figure 
116 shows a converg- 
ing lens used as a 
magnifying glass. 
The lens is held at a 




Figure 1 1 6. 



Image 
A Pocket Magnifying Glass.- 



DIFFUSED LIGHT 



125 



distance less than F, and a large, erect, virtual image is 
obtained. 

The magnifying glass is often used as a reading-glass, It 
is also used by biologists for examining plants and small 
insects. 

150. Diffused Light. — Figure 117 (b) shows a beam of 
light falling on an irregular surface. Part of the light is 
absorbed, but the rest 
is reflected according 
to the law of reflection, 
making the angle of 
reflection equal to the 
angle of incidence. 

Since the surface is 
irregular, the light is reflected in every direction. These 
reflected rays are called diffused light. 





a 

Figure 1 1 7. 



Explaining Diffused Light. 




Figure 118. 



The Automobile Head-Light Lens Diffuses 
the Light. 



It is by diffused light that we see all bodies which are not 
incandescent, that is, light giving. An object such as a 



126 REFRACTION AND LENSES 

perfect mirror (a, Figure 117), which reflects the light in 
parallel rays, cannot be seen. This is illustrated by the 
fact that a person will sometimes walk into a mirror and 
not know it until he has struck it. One looking into the 
mirror does not see the mirror, but only the objects re- 
flected in it. 



CHAPTER XI 
ILLUMINATION AND CANDLE POWER 

151. Intensity of Illumination. — -One often desires to 
speak of the amount of light falling on a surface. To ex- 
press this, the term intensity of illumination is used. 

The intensity of illumination is the light energy per unit 
area. 

To illustrate this definition, suppose you had a slice of 
bread and were to spread a serving of butter upon it. The 
butter would be of a certain thickness. Now, if an equal 
serving of butter were spread on several slices, its thickness 
would be much less. This is true of light. 

When a certain amount of light falls on a definite area 
the intensity of illumination is a certain amount; but if 
the same light were spread over a larger area, the intensity 
would be less. 

Every one has noticed that the greater the distance from 
the source of light, the weaker the light becomes. This is 
stated in the following law : 

The intensity of illumination is inversely proportional to 
the square of the distance from the source of light. 

To prove this law, suppose a cardboard (a, Figure 119) 
is placed before a light (L), the cardboard having a small 
hole in it. A second cardboard (6) with a square hole, one 
inch on a side, cut in it is placed one foot from a. A third 
cardboard (c) is placed two feet from a. 

127 



128 



ILLUMINATION AND CANDLE POWER 



** 



Now, the light coming through the square hole in b falls 
on a certain area on c. 

From the figure it will be seen that the side of the illu- 
minated square on c is twice the side of the square in b. 




a b c 

Figure 119. — Explaining Law of Intensity of Illumination as the 
Distance Varies. 

Thus the light falls on an area at c, which is four times as 
large as on b ; etc. 

Thus the area on which the light falls is directly proportional 
to the square of the distance from the source. 

Since the intensity of illumination is inversely propor- 
tional to the area, it is inversely proportional to the square 
of the distance from the object under consideration to the 
source of light. 

, This law can be applied to reading. If your book is 
three feet from the lamp the printed pages will be illu- 
minated four times as strongly as if it were six feet away; 
nine times as strongly as if it were nine feet away; and 
10,000 times as strongly as if it were 300 feet away. This 
shows you why it is so important to get close to the light 
to get proper illumination. 

152. Candle Power. — We have discussed the intensity 
of illumination of objects lighted by some source other than 
themselves ; but it is often desired to express the brightness 



MEASUREMENT OF CANDLE POWER 



129 



of the source of light itself. The unit used for this is called 
the candle power. 

One candle power is the light given by a standard candle 
burning wider specified conditions. 

The standard candle is made of sperm oil, weighs ^ of 
a pound, is usually wrapped in tinfoil, and burns at the 
rate of 120 grains per hour. 

It will be seen immediately that the unit candle power is, 
at best, a poor unit, because no matter how much care is 
taken to get the conditions the same, a candle will never 
give exactly the same light. It is like using a tape measure 
made of rubber. Nevertheless, this unit is still used for 
want of a better one. 

153. Measurement of Candle Power. — In measuring the 
candle power of a source of light, the light is compared to 
either a standard candle or to 
another light of which the 
candle power is known. To 
make this comparison the 
photometer is used. 

The photometer is a piece 
of paper with a grease spot on 
it. This paper may be either 
placed in a small black box '; 
(Figure 120), or may be put 
in a standard which holds it 
in position. 

To compare two lights, the 
photometer is held between them, at such positions that 
the illuminations on both sides of the paper are the same. 
(Figure 121.) 

This point can be determined, since the grease spot will 




Figure 120. — Cross Section of 
Bunsen Photometer. 



130 ILLUMINATION AND CANDLE POWER 

disappear, or look the same shade on both sides, when the 
correct position is reached. 

By measuring the distance (d x ) of the unknown light (X) 
to the photometer, and the distance (d 8 ) from the known 




S X 

Figure 121. — Comparing Two Lights by Use of Photometer. 

standard (S) to the photometer, the candle power of X can 
be calculated. 

'-©■• 

The candle power of a few sources of light are as follows : 

Carbon Lamp . . . . . . about -f- c. p. per watt 

Tungsten Lamp about 3- c. p. per watt 

Nitrogen Lamp about 1 c. p. per watt 

Mercury Vapor Lamp .... about 1 c. p. per watt 

Arc Light about 1 c. p. per watt 

154. Problems in Illumination. — The problem of the 
proper illumination of different kinds of buildings, streets, 
etc. is an important one. It is one which cannot be an- 
swered or solved in this text. Only a few suggestions as 
to its importance and application can be made. 

In the home, care should be taken to have lights placed 
in the proper positions. Also, candle power of lamps to be 
used is largely determined by the decorations of the room. 

For the kitchen, two lamps are usually needed : one above 
the sink, and one above the stove. Forty-watt tungsten 
lamps are, as a rule, a good rating. 



PROBLEMS IN ILLUMINATION 131 

A bedroom should have at least a 40-watt tungsten. 
This should be hung above the dresser or dressing table, 
and not from the center of the ceiling. 

The bathroom should have two lamps, one on each side 
of the mirror. Twenty-five-watt tungstens are sufficient. 

The lamps in the living rooms, library, etc., cannot be 
specified, but should be placed so as to be most convenient 
and at the same time bring out the desired effects of the 
decorations. 

It is astonishing what different effects may be obtained 
by different lightings of the same piece of statuary. The 
same is true of paintings. 



CHAPTER XII 

COLOR 

155. Dispersion. — If a ray of white light be passed 
through a glass prism (Figure 122), it will be refracted and 
at the same time will be broken up into a band of seven 
colors, in the order of violet, indigo, blue, green, yellow, orange, 
and red (vibgyor contains the initials of the colors in the 




Figure 122. — White Light Passing through a Prism. 

regular order). This breaking up of white light is called 
dispersion, and the band of seven colors is called the solar 
spectrum . 

156. Cause of Different Colors. — At the beginning of 
our discussion of light we said that light is a wave motion 
in the ether. Different wave lengths give differently colored 
light ; that is, the color of the light depends upon the wave 
length, just as the high tones in sound have different wave 
lengths from the low tones. 

132 



THE ACHROMATIC LENS 



133 



The violet rays are the shortest waves (about .000033 cm.) 
which the eye can see, while the red rays are the longest 
(about .000081 cm.), the other colors falling in between, 
in the given order. 

When a piece of iron is heated, it first becomes red hot 
and later white hot. As more heat is applied, the molecules 
vibrate faster and faster, sending out shorter and shorter 
wave lengths as well as the longer ones, thus producing all 
the colors of the spectrum. Just as white light can be 
broken up into all these colors, so they now combine and 
make the iron look white. Hence the term white hot. 

This same thing can be noticed in the filament of an 
electric lamp when it is partially lighted, then fully lighted. 

157. The Achromatic Lens. — When a lens is made of 
one piece of glass, it does not refract all colors equally; in 
other words, dispersion takes 

place. This makes it impos- 
sible to get a perfect focus with 
this kind of lens. 

To correct this defect, lenses 
are made of crown and flint 
glass. (Figure 123.) The dis- 
persive effect of one glass 

counteracts the dispersive effect of the other, but the rays 
are still refracted, thus producing a perfect focus. This 
kind of lens is called achromatic — without color. These 
lenses are very expensive and are used only in high-priced 
cameras, microscopes, and other optical instruments. 

158. Transparent, Translucent, and Opaque Objects. — 
Objects are divided into three classes, according to their 
ability to transmit light. 

Transparent objects are those which transmit light in 




Figure 123. — An Achromatic 
Lens. 



134 COLOR 

parallel rays ; and thus objects can be seen in detail through 
them. 

Translucent objects are those which transmit light, but not 
in parallel rays, so that objects cannot be seen in detail 
through them. Light after coming through a translucent 
object is diffused. 

Opaque objects are those which shut off the light entirely. 

Air, clear plane glass, clear water, etc., are examples of 
transparent objects. 

Snow, cracked ice, frosted glass, thin paper, etc., are 
examples of translucent objects. 

Wood, iron, stone, etc., are examples of opaque objects. 

159. Color of Opaque Objects. — No object, unless it is 
self-illuminated, has color. It gets its color from the light 
that falls on it. 

The light that falls on it is either absorbed or reflected, 
the object taking on the color of the light that it reflects. 
Thus a red dress is not red at all, but merely absorbs all 
colors that fall on it except red, which it reflects, thus giving 
it the apparent red color. 

This same red dress in a perfectly dark room would be 
black. It would also be black, or purplish (depending upon 
the shade of red), if held in the light of a sodium flame, 
because this light contains only yellow, and so there would 
be no red to be reflected. 

160. Dyes. — A dye is a substance which may be made 
to stick between the fibers of another object and thus give 
the object an apparent color by reflecting that colored light. 

Cloth is usually dyed by placing it in a liquid containing 
certain substances which enter the cloth and stick between 
the fibers after the dye has dried. If it is a good dye, it is 
of such a nature that these particles cannot be washed out, 



APPLICATION OF COLORED OBJECTS 135 

causing the cloth to fade. A good dye should also be un- 
affected by sunlight. 

When a cloth fades, the small particles are either washed 
out or are so changed chemically that they will not reflect 
the desired color. 

161. Paints. — Paints are different from dyes in that 
they are colored pigments which are spread over the surface 
of an object, instead of going in between the fibers. The 
color of the paint is determined by the colored light which 
the pigments reflect. 

162. Color of Transparent and Translucent Objects. — 
Transparent and translucent objects get their color from 
the light which they transmit. A green glass is green be- 
cause it absorbs all other colors and transmits the green. 
Objects viewed through green glass appear green because 
that is the only kind of light that gets through. 

Colored glass is made either by putting the coloring 
material in the glass when it is manufactured, or else by 
covering the glass with a film of gelatin containing the 
coloring-matter. 

163. Application of Colored Objects. — From the preced- 
ing topics it is seen that the color of an object depends upon 
two things : the kind of light falling on it, and the color which 
it reflects or transmits. 

The knowledge of this fact is applicable in the selection 
of dress goods and in the illumination of pictures and other 
decorations. 

In selecting dress goods, the selection should be made in 
the same kind of light as that in which the dress is to be 
worn. For example, if a piece of goods is selected in arti- 
ficial light, it should be worn in the same kind of artificial 
light, for it may be of an entirely different color when viewed 



136 COLOR 

in daylight. As an exaggerated example, a bright red piece 
of cloth in daylight would appear dark purple or black in 
the light of a mercury vapor lamp. This is because there 
is no red light given off by the mercury lamp, and conse- 
quently the material has no red to reflect. 

In the same way a blue piece of goods in daylight looks 
black under a carbon lamp, since the carbon lamp gives cff 
very little blue light. 

The same application can be made in illuminating pic- 
tures, wall paper, draperies, etc. These decorations will 
take on an entirely different color when placed under differ- 
ent colored lights. 

A lamp has recently been put on the market, called the 
" day-light lamp." It is given this name because the rays 
sent out by it contain the same colors, and in the same 
proportion, as are found in sunlight. Most large stores now 
have these lamps, so that goods selected in this light will 
have the same color in sunlight. 

164. The Three Primary Colors. — It was found that by 
passing white sunlight through a prism it could be dispersed 
into seven colors. 

Each of these colors is elementary ; that is, it cannot be 
broken up into parts or other colors. This would lead us 
to believe that to get white light we must mix these seven 
colors, and this is partially true. 

A mixture of these seven colors in the right proportions 
will give white light, but white light can also be obtained 
by the mixture of three elementary colors : red, green, and 
violet. More than that, any color whatsoever can be ob- 
tained by the correct proportions of these three colors. 

For this reason the three colors red, green, and violet are 
called the primary colors of light. 



MIXING COLORED LIGHTS 137 

165. How We See Color. — Referring back to the topic 
on " The Eye " (§ 142), it will be found that the retina, 
the inner lining of the back of the eye, is composed of 
countless numbers of nerve-endings or cells, that these 
cells are divided into three classes, but are all intermingled, 
so that even the smallest spot on the retina has all three 
kinds of cells. 

One of these classes of cells is affected by red light, and red 
only ; another is affected by green light, and green only ; 
while the third class is affected by violet light, and violet 
only. 

Xow, when an image falls on the retina, these cells are 
affected by the light that strikes them. Where only red 
light falls, only those corresponding nerve cells are affected ; 
the same for green ; and the same for violet. 

If a light such as yellow, which is composed of both red 
and green, falls on a spot on the retina, both those corre- 
sponding kinds of cells are affected. 

When these cells are affected, impulses are sent to cor- 
responding nerve cells in the brain, and we become con- 
scious of those certain kinds of light falling on their respective 
positions on the retina. Thus we know the shape of the ob- 
ject and also its color. 

166. Mixing Colored Lights. — It has been noted that 
lights of different colors may be mixed. When this is 
done, the result is the combined effects of all he lights each 
taken separately. This is called the additive method. 

Thus, when the correct proportions of red light and green 
light are superimposed, the result is the sum of the red and 
green effects, which gives a yellow. Likewise, any color 
whatsoever may be produced by adding the proper portions 
of the three primary colors. 



138 



COLOR 




Figure 124. — Colored 
Disks. 



The above statements can be experimentally illustrated 

by the use of colored disks on a turning table. (Figure 124.) 

By placing these disks on the spindle, 

one over the other, in such a manner 

that a certain portion of each disk is 

visible, and then by turning the disks 

at a rapid rate, an apparent mixture of 

these colors is attained. The mixing 

is done on the same principle as the 

moving-picture (§ 147), each color effect 

being superimposed upon the retina of 

the eye before the other color effects disappear. 

167. Tints and Shades. — A tint of a certain color is 
produced by adding that color to white. In the same way 
shades of a color are produced by mixing that color with 
black. 

168. Colored Pigments. — Colored pigments are used in 
paints and dyes, and are small particles of matter of such a 
nature that they reflect certain colors. 

169. Mixing Pigments. 
— Mixing pigments to 
produce color is called 
the subtr active method. It 
is called subtractive be- 
cause the color that is 
given out after mixing the 
pigments is that which is 
left after the pigments 
have absorbed their char- 
acteristic colors. Thus 

Figure 125 illustrates the , 

adding of red and yellow, and Violet Lights. 




MIXING PIGMENTS 



139 



White 



Light 



Light Gray 



-Light Gray- 



Neutral Gray — 



-Dark Gray- 



Dark 



Dark Gray 




Figure 126. — Mixing Six 
Different Colored Pig- 
ments. 



yellow and violet, violet and red, and red, yellow, and violet. 

It will be seen that the resulting colors are, respectively, 

orange, green, purple, and black. 
The three kinds of pigments, 

red, yellow, and violet, are called 

primary, because by adding them 

in the right proportion black is 

obtained. 

Each of the 
three kinds of 
pigments absorbs 
certain colors, 
giving back only 
its characteristic 

color. When the three kinds are mixed 
together, no color is given back, for 
what one gives back the others absorb. 
This produces the absence of color, or 
black. 

Figure 126 is a diagram illustrating the 
mixing of six kinds of pigments, and the 
resulting effects. Thus a mixture of red 
and orange gives a red-orange ; a mix- 
ture of orange and yellow gives an 
orange-yellow, etc. 

Opposite colors, such as red and green, 
orange and blue, yellow and violet, are 
called complementary colors, because if 
the one is taken from white the other is 
the result. For example, if red is taken 
from white, green is the result, etc. 
Figure 127 is a diagram showing how 



Black 
Figure 127. — Dif- 
ferent Shades of 
Gray. 



140 COLOR 

to obtain different shades of gray. Half white and half 
black give what is called neutral gray. Three-fourths black 
and one-fourth white give a dark gray. Three-fourths white 
and one-fourth black give a light gray. Greater quantities 
of black than three-fourths give a dark dark-gray. Greater 
quantities of white than three-fourths give a light light-gray, 
etc. Thus any shade from white to black may be obtained 
by a mixture of the proper proportions. 

170. Limitations of Color Nomenclature. — We use the 
terms red, blue, green, pink, pea-green, sky-blue, etc., very 
freely, as if they were definite in meaning. The fact of 
the matter is, they are very indefinite. 

For example, could you tell exactly what color to get if 
you were sent to buy sky-blue or pea-green silk? The 
trouble is, our terms are not definite, but cover a wide 
range of color. We still use these indefinite terms for 
want of better substitutes. 

171. Harmony of Color. — In music certain tones sound 
pleasing when given together. The law governing the 
combining of these tones is called harmony. In the case 
of colors it is just as true that certain combinations of color 
are pleasing, while others are not. We speak of this as the 
harmony of color. 

So far there are few set rules or laws governing these 
combinations, since they are left to the taste of the in- 
dividual. What looks well to one individual may be almost 
shocking to another. 

It is true, however, that the following simple rule can be 
followed, and that, in general, it will give a pleasing com- 
bination. All colors harmonize with black and with white. 

172. Half-tone Picture Printing. — In half-tone picture 
printing a negative is obtained from either the object itself 



HALF-TONE PICTURE PRINTING 



141 




or from a photograph, in exactly the same manner as in 
photography. 

Instead of printing on a sensitized paper as in the case 
of a photograph, the negative is placed over a sensitized 
plate of copper or other metal, and the picture is printed on 
this. 

The copper plate is made sensitive by a covering of gelatin 
sensitive to light, just as in the case of the paper. 

Before the printing on the metal plate is begun, two glass 
screens (a and b, Figure 128) are placed, one over the other, 
between the negative and 
the plate. These screens 
are usually ruled with 
from 100 to 150 parallel 
lines to the inch, and, 
when placed over one 
another (c), the lines of 

one are perpendicular to the lines of the other; the lines 
being scratches which shut off light. 

In printing, the light shines through the light part of the 
negative, turning the sensitive gelatin on the metal plate 
black, and making it insoluble. The rest of the gelatin 
is unaffected, and when " washed " dissolves, leaving the 
black, insoluble part on the plate. The lines of the screens 
appear as clean lines on the plate. 

This metal plate is then subjected to an acid bath which 
etches, or eats away the unprotected part of the plate, leav- 
ing the part covered with gelatin " raised " or level with 
the original surface. 

After scraping off this gelatin the plate may be inked 
and used for actual printing of pictures in books, magazines, 
or newspapers. 



a 

Figure 128 



Light Screens. 



142 



COLOR 



Since most printing is done from rolls, the impression 
may be transferred from the metal sheet to the rolls by the 
electrotype method. (§ 280.) 

By referring to Figure 129 it can be seen why the metal 
plate will produce a picture which is the exact likeness of 
the object. 

The light part of the negative represents the dark part 
of the object. The raised part of the metal plate represents 
the light part of the negative or the dark part of the object, 






Object Negative Plate 

Figure 129. — Diagram Showing Object, Negative, and Plate in 
Half-tone Picture Printing. 



the lines of the two screens appearing as depressed parts on 
the metal plate. 

Now, when the metal plate is inked and a picture is 
printed with it, the raised portion is the only part that 
prints, thus reproducing the dark parts of the object in ink. 
The lines are to keep the ink from " running." They do 
not show, except upon close examination, in the printed 
picture. 

173. The Three-color Printing Process. — The half-tone 
picture printing process, discussed in § 172, gives a picture 
in light and shadow only. This process has been enlarged 
upon, and now pictures in actual colors can be printed by 
what is called the " three-color process." This process is 



THE THREE-COLOR PRINTING PROCESS 143 

used to print the colored cover designs and colored advertise- 
ments used so much in the better magazines. 

In this process three negatives are taken through three 
separate light filters. The three filters consist of three plates 
of glass stained violet, blue-green, and orange, respectively. 

These filters are placed in front of the camera, one at a 
time, when the three negatives are taken. The negatives 
are developed and printed on three separate metal plates, 
as in the half-tone process. 

These plates, or their reproduced rolls, are then inked, — 
the one corresponding to the violet filter with yellow ink, the 
one corresponding to the blue-green filter with red-orange 
ink, and the one corresponding to the orange filter with blue 
ink. Then all three are successively printed on the same 
sheet of white paper. The result is a picture of the object 
in actual colors, or at least approximating the actual colors, 
the degree of accuracy in colors depending on the trueness 
of the colors of the filters and inks used. 

The reasons why this process gives the actual colors are 
as follows : 

In the first place, the negative taken with a violet filter 
has dark spots only where the violet light strikes, and so the 
corresponding metal plate has depressed spots representing 
the violet of the object. 

Likewise, the metal plate corresponding to the blue-green 
filter has depressed spots representing the blue-green of the 
object, and the metal plate corresponding to the orange filter 
has depressed spots representing the orange of the object. 

Xow, the three colors, violet, blue-green, and orange, con- 
tain all the colors of white light, and so the depressions in 
the three metal plates represent all the actual colors of the 
object. 




144 COLOR 

The plate corresponding to violet in the object, covers all 

the rest of the white paper with yellow, the complementary 

pigment of violet. Likewise, the plate corresponding to 

blue-green in the object covers all the rest of the white paper 

with red-orange, and the plate corresponding to orange in 

the object covers all the rest of the 

white paper with blue. The spots 

with yellow ink reflect all colors but 

violet, or, in other words, blue-green 

and orange. (Figure 130.) Also, 

the spots with red-orange ink reflect 

all colors but blue-green, or in other 

words violet and orange. 

Figure 1 30. - Diagram. Therefore a spot covered by 

yellow and red-orange inks reflects 

only orange. Also a spot covered by yellow and blue inks 

reflects only blue-green, and a spot covered by red-orange and 

blue inks reflects only violet. 

This makes the printed picture reflect the actual colors of 
the object in the correct positions and amounts. 

Review Problems 

1. What is the theory of the nature of light? 

2. When is a body luminous? 

3. Why can you see a body which is not luminous ? 

4. What is the velocity of light ? 

5. Explain Roemer's method for determining the velocity of light. 

6. Give two comparisons which will show the magnitude of the 
velocity of light. 

7. Give the law cf reflection. 

8. Does your right hand appear to be the right hand of your image 
in a plane mirror? 

9. Construct the image in a plane mirror. Describe the image. 



REVIEW PROBLEMS 145 

10. Construct the image in a concave mirror, (a) when object is 
beyond center of curvature, (6) when object is at center of curvature, 

(c) when object is between center of curvature and principal focus, 

(d) when object is at principal focus, (e) when object is between prin- 
cipal focus and mirror. 

11. Give two uses of the convex mirror. 

12. Give two uses of the concave mirror. 

13. Explain why refraction takes place. 

14. Give five applications of refraction. 

15. Construct the image in the five different settings of the convex 
lens. 

16. Give an application of each of the five settings of the convex 
lens. 

17. Explain how a photograph is made. 

18. What is diffused light? 

19. What produces color in a light ? 

20. Explain why an opaque object has a certain color. 

21. Explain why a stained glass has a certain color. 

22. Why can you not rely on colors chosen by artificial light ? 

23. What application has color to the decorating and lighting 
of a home? 

24. Explain why shadows play an important par. in the proper 
illumination of a room. 

25. How are half-tones made ? 

26. What is a tint? What is a shade ? 

27. What is meant by the " additive method " ? 

28. What is meant by the " sub tractive method " ? 

29. What is the difference between a dye and a paint? 

30. What causes a colored piece of goods to " fade " ? 



CHAPTER XIII 
MAGNETISM 

174. Properties of Magnetism. — We do not know just 
what magnetism is, but we do know many things about it. 
For centuries people have known of a peculiar kind of ore 
called " lodestone," which has the property of attracting 
iron. The " lodestone " is said to have magnetism, and the 
best definition we have is : Magnetism is the property some 
objects have of attracting iron. An object which has mag- 
netism is said to be a magnet. 

175. Poles of a Magnet. — If a magnet be thrust into a 
box of iron filings, the filings will cling to the ends of the 
magnet, and will appear to be attracted to one point near 
each end. This point is called the pole of the magnet, and 
is located inside the iron some distance from the end. The 
pole of a magnet is the point at which all the force of attraction 
is centered. 

A magnet has two poles, one near each end, called north 
(N) and south (S). It is unfortunate that they were named 
" north " and " south," for we are apt to confuse these 
terms with direction. A magnet may be placed in any 
position, and yet its poles remain the same, regardless of 
direction. For example, a magnet may be placed in an 
east and west position, and yet its poles are called N and S. 
A magnet may be easily placed so that its N-pole is on the 
south end (direction) of the magnet. 

146 



FIELD OF A MAGNET 147 

176. Law of Attraction and Repulsion. — If a magnet is 
suspended at its middle by a cord, or balanced on a pivot, 
and another magnet is brought near it, the end of the first 
magnet is either attracted or repelled by the other magnet. 

If the X-pole of one comes near the S-pole of the other, 
they are attracted, and if free, will swing together. But if 
the S-pole of one magnet comes near the S-pole of the other, 
they are repelled, and if free will swing apart. Thus we 
have this law : Unlike poles attract and like poles repel. 

177. The Earth a Magnet. — The earth itself is a huge 
magnet, one of its magnetic poles being about 1000 miles 
from the geographical north pole, while the other magnetic 
pole is at a similar distance from the geographical south pole. 

A magnet suspended so that it is free to swing in a hori- 
zontal plane will come to rest in a north and south position. 
This is due to the magnetic attraction of the earth. The 
pole that swings towards the north is called " N-pole," 
while the one that swings towards the south is called " S- 
pole." At the time the poles were named, people did not 
know that magnets would ever be used for anything except 
to tell direction, and the names " N " and " S " seemed 
appropriate. 

But now the names are confusing. A N-pole is the pole 
that points north when the magnet is free to swing, but by 
the " law of attraction " unlike poles attract ; therefore the 
magnetic pole near the north geographical pole is really a 
" S " magnetic pole. Likewise the " N " magnetic pole of 
the earth is in the south. 

178. Field of a Magnet. — We have seen that a magnet 
will attract iron filings even when they are not touching it. 
What is it that harnesses the iron filings to the magnet, 
since we cannot see, or feel, anything between them? 



148 



MAGNETISM 



Evidently there is some force in the space about the mag- 
net. This space is called the " magnetic field," and is said 
to be filled with " lines of force" 




Figure 131. — Field about a Bar Magnet. 

Just what these lines of force are no one is able to explain ; 
and for want of a better name they are said to be strains in 
the ether. 

If a piece of paper is placed over a bar magnet and iron 
filings are sifted on it, the filings will arrange themselves in 
lines as shown in Figure 131. 






Figure 132. — Arrangement of Mole- 
cules in a Piece of Iron Not Mag- 
netized. 



Figure 133. — Diagram 
of Balanced Forces 
in a Piece of Iron 
Not Magnetized. 



THEORY OF MAGNETISM 



149 



179. Properties of Lines of Force. — Whatever the lines 
of force are, they have three known properties : 




Figure 134. — Arrangement of Mole- 
cules in a Magnetized Piece of Iron. 



Figure 135. — Unbal- 
anced Forces in a Mag- 
netized Piece of Iron. 



1. They have direction and always come out of a X-pole 
and go in at a S-pole, completing a loop inside the magnet. 




Figure 136. — How to Magnetize a Piece of Iron. 

2 They have a tendency to contract, like rubber bands, 
and will contract until they are zero in length. 

3. They repel one another laterally. 

130. Theory of Magnetism. — Some substances are said 
to be magnetic, while others are non-magnetic. Magnetic 




Figure 137. — Field between Two Unlike Poles. 

substances are substances whose molecules have N- and S- 
poles, while non-magnetic substances are those whose mole- 
cules do not have N- and S-poles. 



150 



MAGNETISM 



Iron is the most magnetic substance, while cobalt and 
nickel are only slightly magnetic. Most substances, such as 
wood, glass, copper, brass, etc., are non-magnetic. 




Figure 138. — Field between Two Like Poles. 



The fact that iron is magnetic does not necessarily mean 
that a piece of it is a magnet. It must first be magnetized. 

181. Difference between a Magnetized Piece of Iron and 

One Not Magnetized. — 
In a piece of iron that is 
not magnetized the mole- 
cules have their N-poles 
and S-poles pointing in 
various directions (Figure 
132), and the effect of 
some molecules neutral- 
izes the effect of others. 
It is like several boys 
pulling in all directions 
upon a post. (Figure 
133.) The pull is bal- 
anced and there is no 

„-. r- effect on the post. 

Figure 139. — Field about a Horse- m r 

shoe Magnet. But in a piece of iron 




HOW TO MAGNETIZE A PIECE OF IRON 



151 



which is magnetized, the molecules are all in order ; so that 
all the S-poles point to one end, and all the N-poles to the 
other. (Figure 134.) 

In this case the effect of each molecule helps the effect of 
every other, and one end of the bar becomes a N-pole and 
the other end the S-pole. To 
illustrate this as before, all the 
boys pull in the same direction. 
(Figure 135.) 




hiGURE 140. — Field about a 
Horseshoe Magnet Having a 
Bar of Soft Iron in Front 
of Poles. 

182. How to Magnet- 
ize a Piece of Iron. — 

To magnetize a piece of 
iron, place it in a mag- 
netic field so that the 
lines of force run through 
the iron. This lines the 

molecules up as in Figure 136, magnetizing the iron. 

If it is a piece of tempered steel that has been magnetized, 

the molecules will keep their positions, and the steel will hold 



Figure 141. — Field about a Horse- 
shoe Magnet Having a Disk of Soft 
Iron in Front of Poles. 



152 MAGNETISM 

its magnetism, because the molecules cannot fall back out of 
line. This is, then, a permanent magnet. 

If the piece of iron is soft and not tempered, the molecules 
become disarranged as soon as the magnetic field is removed ; 
and it loses its magnetism. This is a temporary magnet. 

183. Characteristic Fields. — The following drawings 
show the direction of the lines of force in several cases. 
(Figures 137, 138, 139, 140, 141.) 



CHAPTER XIV 



ELECTRICITY 



184. Relation of Electricity to Magnetism. — Before 
studying the subject of electricity we spent some time on 
magnetism, because magnetism and electricity are very 
closely related. We shall now find how necessary magnetism 
is to the production of electricity. 

The question just what electricity is, has never been satis- 
factorily answered. The latest theory is that it is some kind 
of strain in the ether, and that the strain will move along 
a wire, producing a current of electricity. 

Anything which will transmit elec- 
tricity from one place to another is 
called a conductor. 

185. Generation of Electrical Pres- 
sure. — It has been found that if a 
conductor is moved in a magnetic 
field so that it cuts the lines of force 
electrical pressure is produced, or is 
said to be generated. 

In Figure 142 we have a permanent 
magnet with the lines of force shown 

coming out of the X-pole. A copper wire, or rod, is held 
in this magnetic field and moved across the lines of force. 
This generates electrical pressure in the conductor. 

153 




Figure 142. — Generat- 
ing Electrical Pressure. 



154 ELECTRICITY 

If a complete circuit is made from one end of the bar to 
the other, a current of electricity w'll flow. 

The thing that produces the pressure is cutting lines of force 
with a conductor. This, then, is one of the fundamental 
principles to learn about electricity. Whenever lines of force 
are cut by a conductor, electrical pressure is generated. 

186. Nature of Electrical Pressure. — But just what is 
electrical pressure? Since electricity is an invisible some- 
thing and yet is analogous to the flow of water, we can best 
get a conception of it by comparing it to the flow of water. 

In the case of water, we say there is a pressure of so many 
pounds per square inch. Pressure is the thing that makes 
the water flow when the stop-cock is turned on. The pres- 
sure is there whether the cock is turned on or not, and when- 
ever the water has a chance to flow, the pressure forces it to 
do so. 

Electrical pressure is similar. It is that which makes the 
electrical current flow. There may be an electrical pressure, 
and yet no current (if the circuit is not closed) ; but if there 
is a possibility for the current to flow (as when the circuit 
is closed) the pressure will make it do so. 

The amount of electrical pressure depends upon the rate of 
cutting lines of force ; or, we could say, upon the number of 
lines of force cut per second. 

The direction of the pressure depends upon the direction 
in which the lines of force are cut. 

187. Electrical Current. — The electrical current may be 
compared to the current of water in a pipe. We say the 
current is large or small according to the amount of water it 
will deliver in a certain time. Similarly with electricity, 
the current is the flow of the electricity, and is measured by the 
amount of electricity it will deliver per second. 



THE SIMPLE GENERATOR 



155 



The size of the current depends upon the pressure forcing 
it to flow, and upon the resistance offered to it by the con- 
ductor. 

188. Resistance. — If the water pipe in the above case 
were small, it would be difficult for the water to get through. 
In other words, the pipe would offer a resistance to the flow 
of the water current. The same thing takes place in a wire. 
The resistance is that ivhich tends to hold the current back. 

There are four principal things which affect the resistance 
of a conductor : (1) size, (2) length, (3) kind of material, 
(4) temperature. 

The larger the wire, the smaller the resistance. The 
longer the wire, the greater the resistance. Some kinds of 
material have more resistance than others. For instance, 
copper has less resistance than iron. 

Materials which have a low resistance are said to be good 
conductors. Copper, silver, platinum, and, in fact, nearly 
all the metals are good conductors. Those materials which 
have an exceptionally high resistance are called insulators, 
such as air, wood, glass, mica, rubber, asbestos, etc. 

The temperature affects different materials differently. 
With some, it increases the ^ 

resistance ; and with others '■ ■ "^ ' ' ■ ■ ■ ■ ■ ' 

it decreases it. A carbon 
lamp has less resistance 
when hot than when cold, 
but a tungsten lamp has 
more resistance when hot. 

189. The Simple Gener- 
ator. — Figure 143 shows a 
Joop of wire revolving in a magnetic field. The magnetic 
field is produced by the permanent magnets N and S. The 



S 



r 



Figure 143. — A Simple Generator. 



156 



ELECTRICITY 



lines of force pass from the N-pole across, and into the 
S-pole. The loop of wire is a conductor; and when it 
revolves in this magnetic field, it cuts the lines of force, 
and electrical pressure is generated. 

190. A. C. Simple Generator. — Figure 144 shows a cross 
section of the simple generator. Since it is a cross section, 
the ends of the loop of wire, where it is cut off, are dots. In 




Figure 144. — Cross Section of Simple A. C Generator. 



this discussion we shall mention only one side of the loop of 
wire. 

Suppose we start with the wire at position a and turn it 
around, or revolve the loop at uniform speed. 

At position a the wire is moving parallel to the lines of 
force, and so does not cut any. Therefore there is no pres- 
sure being produced. This can be shown on the curve 
(Figure 145) at position a. 

Now let the loop revolve until the same wire is at b. 
Here it is moving perpendicular to the lines of force, and so is 
cutting them at the greatest rate possible. Therefore there 
will be the greatest pressure generated, — shown by point b 
on the curve. 

Now, when the loop revolves so that the wire is at posi- 
tion c, the wire is again moving parallel to the lines of force. 
Again the pressure is zero, — point c on the curve. 



A. C. SIMPLE GENERATOR 



157 



As the loop revolves farther, the wire begins to cut the 
lines of force in the opposite direction ; and so the pressure 
will be in the other direction, or will be negative. When 
the wire reaches position d, it is again moving perpendicular 
to the lines of force, and so is cutting the greatest number 




— Pressure 

Figure 145. — Curve Showing Pressure at Different Parts of the 
Turn of the Armature in an A. C Generator. 



again; and so the pressure is highest, but in the negative 
direction, — point d on the curve. 

When the loop completes the turn, the wire is at the same 
point as when it started, so the effect is the same, — point 
e on the curve. 

Reviewing what has just taken place throughout the turn, 
we find that the pressure started at zero, then gradually 
increased in the positive direction until the loop had made a 
quarter turn. Here the pressure was the highest, but imme- 
diately began to diminish until at the half turn it had died 
down until it was again zero. At this position the pressure 
began to increase, but in the opposite direction, and con- 
tinued to increase until it reached its highest value at the 
three-quarters turn ; then decreased until it reached zero at 
the complete turn. 



158 



ELECTRICITY 




Figure 146. — Photograph of a Hand Generator. 




Figure 147. — Photograph of a 300 Horse Power D. C. Generator. 



SLIP-RIXGS 



159 



Thus we see that the pressure was first in one direction 
for half a turn, and then in the opposite direction for half a 
turn. This is called alternating current pressure, and it 
makes the current flow first in one direction throughout the 
circuit, and then stop and flow in the other direction. 

Alternating Current (A. C.) is an electrical current that 
flows first in one direction and then in the other. 

Direct Current (D. C.) is an electrical current that floics in 
the same direction all the time. 

191. Slip-rings. — From the above discussion we see 
that whenever a loop of wire revolves in a magnetic field, 




Figure 148. — Slip-rings and Where They Are 
Placed on the Armature of an A. C Machine. 



an alternating current is produced in the loop, which is 
called the armature. If this current is taken off just as it is 
produced, the current will be alternating, throughout the 
outside circuit. Current is sometimes taken off by means of 
slip-rings. Slip-rings are two continuous rings of metal put 
on the end of the armature, as is shown in Figure 148. 

The ends of the coil are fastened on these rings, one end 
on one ring and the other end on the other ring. Metal or 
carbon "brushes" rest on these rings and pick the current 
off just as it is made, thus producing an A. C. current in 
the external circuit. 



160 



ELECTRICITY 



192. D. C. Simple Generator. — The D. C. simple gen- 
erator is the same as the A. C. simple generator, except in 
the way the current is taken off. In the 

OA. C. generator it is taken off by slip-rings, 
while in the D. C. generator it is taken off 
by a commutator. 

193. Commutator. — A commutator is the 
same as one slip-ring, except that it is split. 
It consists of two or more segments, as is 
shown by Figure 149. 

This is put on the end of the armature instead of the slip- 
rings. One end of the loop of wire is fastened to one seg- 
ment, while the other end of the wire is fastened to the 
other segment. " Brushes " are placed against these seg- 
ments to take off the current. 



Figure 149. — A 
Commutator Is 
a Slip-ring 
Cut in Parts. 




Figure 150. 



A Large Generator at Niagara Falls, 
Water Turbine. 



Driven by 



COMMUTATOR 



161 



Since the current alternates in the loop of wire, first one 
commutator segment is positive {i.e. the current comes out), 
and then the other. But the brushes are so set that when 
the current changes in 
the loop, the brushes slip 
from one segment to the 
other ; thus one brush 
is always positive, and 
the other is always nega- 
tive. Figure 151 will 
help to show this change. 

In position a, number 






Figure 

Makes A 



b c 

151. — How the Commutator 



C. Become D. C. 



1, commutator-bar is on the right, 
and is negative, while number 2 bar is on the left, and is 
positive. This makes the upper brush positive, and the 
lower brush negative. 

In position b, the coil has turned one-half the way round, 
putting number 1 on the left and number 2 on the right ; but, 
in turning, the current is reversed, so that now number 1 is 
positive and number 2 is negative. This still leaves the upper 
brush positive and the lower brush negative. 

-j- Pressure 




Figure 152. — Curve Showing the Pressure at Differ- 
ent Parts of the Turn of the Armature in a D. C. 
— Pressure Generator. 



In position c, the conditions are the same as in a. This 
shows that the current always comes out of the same brush, 
or has become D. C. 



162 ELECTRICITY 

194. Curve for D. C. — Referring back to Figure 145, the 
curve for the simple generator, we see that the curve changes 
somewhat when the commutator is put on. It changes to 
the curve on the preceding page. (Figure 152.) 

The first half-turn is the same, but the second half-turn 
becomes positive, due to the fact that the brushes slip from 

one bar to the 

1 <f\ other at the same 

time the current 

changes direction. 

195. A Pulsat- 

Figure 153. — Cross Section of a Generator . _ __ 

with 3 Coils. ^g D. C. Made 

Steady. — ■ From 
the curve (Figure 152) we see that the current rises and 
falls with each half-turn of the loop of wire. This is what 
is called a pulsating current. But if, instead of one coil of 
wire, several coils are put on, as in Figure 153, then the 

-J- Pressure 





'</ V V V V y/ V V V N 



K 1 IY2 Turns 



Figure 154. — Curves Showing Pressure from Three Coils. 
The resulting pressure is represented by the tops of the curves. 

current becomes steady. The reason for this is easily seen. 
There is never an instant when some coil is not cutting 
the lines of force at right angles, thus constantly keeping 
the pressure at the highest. (Figure 154.) 



CHAPTER XV 



MAGNETIC EFFECT OF AN ELECTRICAL CURRENT 



196. Magnetic Field about a Wire Carrying a Current. — 
We have seen that cutting lines of force by a conductor 
produces electrical pressure. On the other hand, a current 
of electricity, like a magnet, has about it a magnetic field. 

If a wire carrying a current of electricity be passed through 
a cardboard (Figure 155), and iron filings be sifted on the 
cardboard, the filings 
will arrange them- 
selves, in concentric 
circles, about the 
wire. This shows that 
the current has a 
magnetic field, and 
that the lines of force 
are in concentric 
circles. 

To determine the 
direction of these 

lines, use this rule : Grasp the wire with the right hand, the 
thumb in the direction of the current, and the fingers will point 
out the direction of the lines of force. A magnetic needle set 
on the cardboard will also show the direction of lines of 
force. (Figure 156.) 

If a wire carrying a current be held over a magnetic needle, 

163 




Figure 155. — The Field about a Wire 
Carrying an Electric Current. 



164 



MAGNETIC EFFECT OF CURRENT 



the needle will tend to turn at right angles to the wire. (Figure 
157.) The following rule can be used to tell which direction 

the needle will turn: 
Extend the fingers of 
the right hand along the 
wire with the wire be- 
tween the palm of the 
hand and the needle, 
and the thumb will 
point the direction the 
N-pole of the needle 
toill turn. 

197. Current through 

a Helix. — A helix is 

a coil of wire wound 

round and round in a spiral. It may have a core, or it 

may not. Let us use a piece of soft iron for a core. Now, 




Figure 156. — Magnetic Needles Show Di- 
rection of Field about a Wire Carrying 
an Electric Current. 



tf 



Current 



Figure 157. 



Needle 

Magnetic Needle Turns with the Lines of Force. 



when a current is passed through the helix, it makes the 
iron a magnet with a north and a south pole. (Figure 158.) 

The coil would become 
a magnet whether the 
iron were in it or not, 
but the soft iron makes 
the magnet much 
stronger. Why ? 
To determine the north pole of an electro-magnet (for that 
is what the coil is called), use this rule: Grasp the coil with 




Figure 158. — Diagram Showing Posi- 
tions of Poles of an Electric Magnet. 



DOORBELL AND BUZZER 



165 



the right hand with the 
fingers in the direction of 
the current, and the thumb 
will point to the north 
pole. 

Note that the position 
of the north pole is de- 
termined by the direc- 
tion which the current 
takes around the coil. 
The fact that the current 
goes in at one end or the 
other has nothing to do 
with the north pole. 

198. Electro-magnet. 
— For a definition of an 
electro-magnet we can 
give this : An electro- 
magnet is a magnet 

formed by a current passing around, or near, the magnet. 

APPLICATIONS OF THE ELECTRO-MAGNET 

199. Doorbell and Buzzer. — The doorbell is one of the 
most common applications of the electro-magnet. The cur- 
rent is started at the battery (B, Figure 160) ; goes through 
the coils C, C ; then into the vibrator V ; then into the set- 
screw S; then into the push button P; and, finally, back 
into the battery, forming a complete circuit. 

When the push button P is held down, the current flows 
through the circuit, magnetizing the coils C, C. These 
coils then attract the soft piece of iron on the vibrator, pull- 
ing it away from contact with S, and striking the bell with 




Figure 



i59. — Photograph of a 2-Ton 
Lifting Magnet. 



166 



MAGNETIC EFFECT OF CURRENT 



the hammer. As soon as contact is broken, the coils lose 
their magnetism, and the vibrator flies back in contact with 

S, due to the spring in 
the vibrator. As long as 
the button is held down, 
this operation is repeated 
again and again, causing 
a steady ringing of the 
bell. 

A buzzer is simply a 
doorbell with the bell left 
off. The buzzing sound 
is made by the vibrator. 

200. The Telegraph 
Sounder. — The telegraph 
sounder consists of two coils of wire (C, C) and a soft iron 
bar (SI) supported on a pivot (P) in such a manner that a 
spring (S) holds the end of a bar up 
against a screw (D). (Figure 162.) 




Figure 160. — Wiring Diagram cf 
Electric Doorbell. 




Figure 161. — Photo- 
graph of Electric 
Doorbell. 




Figure 162. — Wiring Diagram of 
Telegraph Sounder. 



When a current is sent through the coils C, C by attach- 
ing a battery at A and B, these coiJs become magnets and 
pull the soft iron bar down until it strikes the screw E, 



THE TELEGRAPH SYSTEM 



167 




Figure 163. — Photograph of the 
Telegraph Sounder. 



making a slight sound. The bar is held in this position as 
long as the current flows ; but as soon as the current stops, 
the coils lose their magnetism, and the bar flips back to D, 
making a loud click. By 
means of these sounds, 
the operator is able to 
read the message. 

201. Telegraph Relay. 
— The telegraph relay 
merely uses the electro- 
magnet to close another 
electric circuit. 

The main current is 
sent through coils C, C 

(Figure 164) by connecting the main line to A and B. 
This magnetizes the coils, and they attract the soft bar of 
iron SI, pulling it up into contact with screw E. This 
completes the circuit between C and D, the binding- 
posts for the local circuit. 

202. The Tele- 
insulated graph System. — 
We have just 
learned the con- 
struction of the 
sounder and re- 
lay, so now we 
will see how they 
are put to use in 
the telegraph 
system. 
Figure 167 shows a system through three cities. At 
Chicago the main wire is grounded ; then a battery (B) is 





El 



\\ \~\c\V\ 
| ["V\pV \. " 




Figure 164. — Wiring Diagram of the 
Telegraph Relay. 



168 



MAGNETIC EFFECT OF CURRENT 



put in ; and also a key (K) and a relay (R). Next, the wire 
runs to Toledo ; and again a key and a relay are connected 

in series with the line. It 
goes then to Cleveland, 
where still another key, 
relay, and battery are put 
in. Then the wire is 
grounded. This com- 
pletes the main circuit. 
Tracing the circuit, we 
start at the ground at Chicago, go through the battery, 
relay, and key to the key, and relay at Toledo, then through 




Figure 165. — Photograph of the 
Telegraph Relay. 




Figure 166. — Photograph of a Telegraph Key. 

the key, relay, battery, and ground at Cleveland, returning 
through the ground to Chicago. 

Off each relay is run a local circuit, in which are a battery 




Figure 167- — Wiring Diagram of a Three Station Telegraph System. 



STREET CAR CIRCUIT-BREAKER 



169 




Figure 1i 



8. — "Wiring Diagram of 
Electric Clock. 



and a sounder. The relay closes the local circuit ; and the 
battery sends a current through the sounder, making it click. 

Note that the current in 
the main line never goes 
through the sounder. 

203. The Electric Clock. 
— Very often it is desired 
to have several clocks run 
exactly together ; in other 
words, to be controlled by 
a master-clock. This is 

accomplished by the so-called electric clock. (Figure 168.) 
The clock consists of a pair of coils (C, C) so arranged 
that when an electric current passes through them they 
turn the soft iron (SI) on the pivot (P), making the pawl 
(R) slip down a notch on the ratchet wheel. Then, when 
the current is stopped, the weight (IF) turns the bar back, 
pushing the wheel around one notch. This takes place 
every minute, thus making the minute hand move one 
space on the dial. 

For sending the current through the coils an electric cir- 
cuit is made through the master-clock. The master-clock 
runs a drum (D, Figure 168) on which is a peg (0). The 
peg touches the point S every minute, thus making a com- 
plete circuit through the battery and electric clock. 

204. Street Car Circuit-breaker. — As a safety device a 
so-called circuit-breaker is put on street cars. Its purpose 
is to break the circuit whenever the current becomes too 
large. It is constructed as in Figure 169. 

The current from the trolley comes into the point a ; then 
goes through the coil C ; then to the arm A ; and out of 
the contact K by point b. The current makes a magnet of 



170 



MAGNETIC EFFECT OF CURRENT 



K 



I 






H\ 



US I 



Figure 169. 



■Wiring Diagram of Circuit- 
breaker. 



the coil, its strength depending on the size of the current. 
If the current becomes sufficiently strong, it lifts the soft 

a iron bar SI, tripping 
the hook H, allowing 
the spring S to pull 
up the arm A, thus 
breaking the circuit. 
The motorman must 
then reach up and pull 
down the arm again 
before he can start the 
car. 

205. The Annunci- 
ator. — The annunci- 
ator is an instrument 
used in office buildings, in elevators, etc., etc., for the pur- 
pose of telling at what 
place the person calling 
is located. There may 
be any number of push- 
buttons, but the dia- 
gram (Figure 171) shows 
an elevator call-system 
for four floors, or for 
four push-buttons. 

In the annunciator are 
four coils (c, c, c, c), five 
binding-posts {a, b, c, d, 
and e), and the door- 
bell (£). 

From the binding- Fl0URB 17a _ , ,, , ,, C[Pcun , 

posts a, b, c, d run wires breaker. 




THE AUTOMATIC ARC LAMP 



171 



oe 



bat. 



*&- 



<^ 



Lfl-i 



ztrr 



B tz^mr 



OOH 



Figure 171. — Wiring Diagram 
of a Four-point Annunciator. 



through coils 1, 2, 3, 4, respectively, these wires all being 
connected with one wire which runs to the bell and finally 
to the binding-post e. This con- p 

stitutes the internal connection 
of the annunciator. 

The external connections are 
as follows : A battery is attached 
to the binding-post e, and then a 
single wire is run up to all of 
the succeeding push-buttons. 
Then from each push-button re- 
turns a wire to its respective 
binding-post, a, b, c, or d. 
Whenever a push-button is 

pushed, it completes the circuit, through the corresponding 
coil and also the bell. Thus the bell is rung, and the needle 
below the magnet is drawn over, indicating which push- 
button was operated. 

206. The Automatic Arc Lamp. — The automatic arc 
lamp, which is used principally to light our streets and large 
factory buildings, is an application of the electro-magnet. 

This principle is used 
automatically to ad- 
just the carbons, which 
are continually burn- 
ing off. To light the 
arc, the carbons must 
first touch; and then 
must be drawn just the 
correct distance apart, 
and kept there. The 
operation is as follows : 




Figure 172. — Wiring Diagram of an 
Automatic Arc Lamp. 



172 MAGNETIC EFFECT OF CURRENT 

The current flows from the line into coil Ci (Figure 172), 
and then divides. One part goes to the upper carbon, and 
the other part goes to the coil C 2 . 

When the lamp is not lighted, the upper carbon falls 
down and touches the lower one ; thus when the current 
first starts, nearly all of it flows through the carbons, instead 
of through lower coil C 2 , for the resistance of the carbons is 
much less than that of coil C 2 . Thus upper coil Ci is mag- 
netized, but lower coil C 2 is not. This pulls the soft iron 
bar SI up, and also the upper carbon which is attached to it. 

As the carbons are separated, the light is formed, and at 
the same time the resistance of the gap becomes more and 
more, forcing part of the current to flow through coil C 2 . 
Whenever this part becomes strong enough to balance the 
pull of coil C\, the carbons are held stationary. 

207. Other Applications of the Electro-magnet. — Other 
applications of the electro-magnet are the automatic tele- 
phone, the electric gas-lighter, and the electric door-latch. 

The automatic telephone takes the place of the operator 
at the switchboard. The person calling does so by pressing 
on a dial at his transmitter, thus calling the number he 
wishes. No telephone operator is necessary to make the 
connection, as the electro-magnets do it automatically. 

The gas-lighter consists of two electro-magnets, — one 
to turn on the gas and light it, and the other to turn the 
gas off. It is used where it is desirable to turn the gas off 
and on from some other place than at the jet. 

The electric door-latch is used principally in apartment 
houses, and is so arranged that the outer door may be opened 
by pressing a button in any of the apartments. The pressing 
of the button closes an electric circuit, causing an electro- 
magnet to release the latch of the door. 



CHAPTER XVI 
HEATING EFFECT OF AN ELECTRIC CURRENT 

208. Work, Heat, and Electrical Energy. — Work is de- 
fined as a force overcoming a resistance and moving it. 
Work is energy, and so is heat. There are many cases 
where work is changed into heat. If you slide down a rope, 
it burns your hands. Your weight forces you down against 
the friction of your hand on the rope, thus doing work; 
and this work is changed to heat. Again, if a piece of iron 
is hammered, it becomes warm. If you stir cake-dough 
rapidly for some time, it becomes warmer. The work you 
do is transformed into heat. 

The same thing is true when a current of electricity is 
forced through a wire. The pressure is the force ; the cur- 
rent is the thing forced ; and the resistance of the wire is 
the thing that holds the current back. It is just like your 
weight forcing your body down the rope against the friction ; 
and, as in that case, heat is produced. 

Learn this important principle : When an electrical pres- 
sure forces an electrical current through a resistance, heat is 
generated. 

209. Electrical Units. — Electrical quantities are definite, 
just like distance, weight, time, etc.; so it is necessary to have 
units to measure them. 

The following table gives the thing to be measured, the 
unit of measurement, and the letter used to stand for it : 

173 



174 



HEATING EFFECT OF CURRENT 



Thing to be Measured 


Unit 


Letteb 


Pressure 

Current 

Resistance 

Power 

Electrical Energy .... 


Volt 

Ampere 

Ohm 
' Watt 

Kilowatt 

Watt-hour 
. Kilowatt-hour 


E 

I 

R 

W 

Kw 

W-hr. 

Kw-hr. 



It will be noted that power is a new term, and that it has 
two units — watt and kilowatt. The kilowatt is the larger 
unit, and is 1000 watts. 

Electrical power is the time rate of delivering electrical energy. 

The electrical power is found by multiplying the pressure 
by the current ; or 

Watts = Volts X Amperes. 
W = E- I. 
N h f Kl ft — Number of Volts X Number of Amperes 



1000 



or Kw 



EI 

1000 



The electrical energy is found by multiplying the power 
by the time, or 

Watt-hours = Watts X Hours. 

W-hr. = WXt. 

Kilowatt-hours = Kiloivatts X Hours. 

Kiv-hr. = Kw X t. 

The terms electrical power and electrical energy are often 
confused. Be sure to get the distinction. 

Electrical power is the rate of delivering energy. It is 






OHM'S LAW 175 

the pressure at a certain instant X the current at the same 
instant. 

On the other hand, electrical energy is a certain amount of 
energy which is actually delivered. It is not the rate of 
delivering the energy, but is the energy itself. The power 
must work for a certain time to give energy. Which do you 
pay for when you pay your light bill, power or energy? 
Does it make any difference whether a 40-watt lamp burns 

1 hour or 3 hours? 

Problems 

1. What power is being used when a carbon lamp taking .5 ampere 
is placed on a 110- volt circuit? 

2. What is the power used when an iron takes 5§ amperes on 110 
volts ? 

3. State, in words, how to find the power in watts and in kilowatts, 
having given the current and voltage. 

4. Find the cost of running ten 40-watt lamps for 5 hours, if elec- 
tricity costs 10 cents per Kw-hr. 

5. Figure your monthly light bill, if you run, on an average, 4 lamps 
of 40 watts each, three hours each day ; an iron taking 5 amperes for 

2 hours, 4 times a month ; and a motor taking 3 amperes for 1 hour, 
10 times a month. Your lighting circuit is 110 volts, the month has 
30 days, and the price of electricity is 9 cents per Kw-hr. 

210. Ohm's Law. — A great scientist by the name of 
Ohm worked out this very fundamental law, known as 
Ohm's Law: 

Voltage = Current X Resistance, or 
E = I • R. (1) 

Which may also be written : 

I = E (2) 

R 



R = 



E (3) 



176 HEATING EFFECT OF CURRENT 

By these three equations it is possible to find voltage, cur- 
rent, or resistance, if the other two quantities are given. 
Always be sure to choose the one which will answer the 
question to your problem. 

Problems 

1. What current will a lamp take on a 110- volt circuit, if its resist- 
ance is 220 ohms ? -IT" _C^ 

2. What current would the lamp above take if placed on a 220-volt 
circuit ? 

3. What current would a lamp take on a 110- volt and a 220-volt 
circuit, respectively, if its resistance were 44 ohms ? 

4. What voltage is necessary to send 6 amperes through an iron, if 
its resistance is 15 ohms? 

5. What is the resistance of a stove, if it takes 5.5 amperes on 110 
volts? 

6. The resistance of the hea ing-element of an iron increases when 
it gets hot. When does it take more current, hot or cold ? 

7. A carbon lamp takes .5 ampere on a 110-volt circuit, while a 
tungsten takes .315 ampere on the same circuit. Which one has the 
higher resistance, and how much ? 

8. A dimmer on a lamp cuts the current down from .315 ampere 
to .2 ampere. What is the resistance of the dimmer, if the lamp is 
on a 1 10- volt circuit ? 

APPLICATION OF HEATING EFFECT OF AN ELECTRIC 

CURRENT 

211. The Carbon Incandescent Lamp. — The carbon in- 
candescent lamp was one of the first electric lamps used, and, 
like all the later lamps, it uses the heating effect of an elec- 
trical current to produce the light, the principle being to 
force a large enough current through a carbon wire to heat 
it to incandescence. 

The lamp consists of a glass bulb from which the air has 
been exhausted. (Figure 173.) Inside the bulb is the carbon 



THE TUNGSTEN INCANDESCENT LAMP 



177 




Figure 173. — Wir- 
ing Diagram of a 
Carbon Lamp. 



wire through which the current must pass. This wire makes 

connection through the end of the bulb by means of small 

pieces of platinum wire, platinum being ^^ 

used because its coefficient of linear ex- 
pansion is nearly that of glass. Other 

materials would cause the glass to break 

when it was heated or cooled. 

The glass bulb is sealed with wax into 

a screw tip, — one end of the wire being 

attached to the side of the tip, while the 

other is attached to a small piece set in 

the middle of the tip. By this means 

the two ends of the wire are insulated from one another. 

Contact is made through the lamp by screwing it into a 

lamp-socket. The screw of the 
socket is one side of the line, and 
the middle portion is the other 
side of the line. 

Carbon lamps can be used on 
either D. C. or A. C. They are 
made for almost any voltage 
(although care must be taken to 
get the correct voltage for the 
circuit in question), and take 
about 3i watts per candle power. 
212. The Tungsten Incandes- 
cent Lamp. — This lamp is con- 
structed like the carbon lamp, 
except that the wire filament is 

made of tungsten instead of carbon. Figure 175 shows 

the tungsten lamp. 

The tungsten has almost replaced the carbon lamp, for it 




Figure 174. — Photograph 
of a Carbon Lamp. 



178 



HEATING EFFECT OF CURRENT 




Figure 175. — Wir- 
ing Diagram of a 
Tungsten Lamp. 



takes about one-third as much electrical power to light it and 
costs very little more for the lamp itself. The objection at 
first to the tungsten lamp was that its 
filament was so fragile. 

The filaments of the first lamps were 
made by grinding the tungsten to a 
powder, making a paste of it and squeez- 
ing it through holes, and then baking it. 
These filaments broke with the least jar. 
Lately manufacturers have learned to 
draw the tungsten metal into wires for 
filaments, and these are even more dur- 
able than the old carbon filaments. 

This lamp can be used the same as the carbon lamp, 
but it takes only about lj watts per candle power. 

213. The Gas-filled Lamp. — The 
gas-filled lamp is a tungsten lamp 
with the bulb filled with a gas, usually 
argon or nitrogen, instead of having it 
a vacuum. The filament is put into a 
more compact coil, so that this lamp 
is used especially with a reflector. 

The gas-filled lamp can be used in 
any place that the carbon or tungsten 
can, and takes about 1 watt per 
candle power. 

Lamps of 100 watts rating, or over, 
are usually filled with nitrogen, while 
lamps of lower ratings are usually 
filled with argon. 

214. The Mercury Vapor Lamp. — This lamp consists of 
a long glass tube, nearly exhausted of air and containing 




UiUiJp 



Figure 176. — Photograph 
of a Tungsten Lamp. 



THE ARC LAMP 179 

a small quantity of mercury. In each end platinum wires 
are sealed, making connections with the electric circuit. 
(Figure 177.) 

To light the lamp, the tube is brought to a horizontal 
position, so that the mercury makes contact from one end 
of the tube to the other. As soon as contact is made, the 
tube is tilted so as to make the mercury flow to one end. 
This breaks contact, and at this point the mercury is vapor- 
ized by the heating effect. This vapor fills the tube, acting 
as a conductor for the current. The current passing through 
the vapor heats it to in- 
candescence, giving off a 
bluish-green light. Some 
mercury vapor lamps are 
lighted by other means 

than tilting, but they all 

. Figure 177. — -Wiring Diagram of a 

use the same principle tor Mercury Vapor Lamp. 

producing the light. 

This lamp is used especially in lighting large buildings, 
such as factories ; for taking photographs ; and for rectify- 
ing A. C. electricity for storage batteries. 

215. The Arc Lamp. — We have already spoken of the 
arc lamp (Figure 172), but since it is an application of the 
heating effect of an electrical current, as well as of an electro- 
magnet, we mention it here. 

The method of lighting is very much the same as in the 
mercury vapor lamp. To light it, the carbons must touch, 
allowing the current to flow through them. Then the car- 
bons must be pulled apart, breaking the electric circuit. 

At the point where the circuit is broken, a high resistance 
is entered. The current flowing through this high resist- 
ance produces heat sufficient to vaporize the carbon at that 




180 



HEATING EFFECT OF CURRENT 




Figure 178. — Dia- 
gram of Electric 
Flat-iron. 



point. This carbon vapor acts as the conductor, and is 

heated to incandescence, giving off a very bright and power- 
ful light. The temperature reaches as 
high as 3500° C. and gives about 1 candle 
power per watt. 

Arc lamps are used to light streets and 
large buildings. They are usually placed, 
100 lamps in a series, on a 5000-volt line, 
taking from 6 to 9 amperes. They will 
work either on A. C. or D. C. 

In moving-picture houses the arc lamp is 

used in the picture machine. These arcs usually take from 

50 to 100 amperes, as a very high candle 

power is desired. 
216. The Electric Flat-iron. — The 

electric flat-iron (Figure 178) is very 

much like the ordinary flat-iron, except 

that it has a heating element and an 

attachment to connect it to the lighting Figure 1 79. — Heat- 
ing Element in an 
System. Electric Flat-iron. 

The heating element is a special kind 
of wire of high resistance wound on an insulator and placed 

inside the iron. 
Very often ni- 
chrome wire is 
wound on a piece 
of mica (Figure 
179), and this is 
then placed be- 
tween sheets of 

Photograph of Electric mica ' The mica 

Flat-iron. acts as an insu- 





Figure 180. 



OTHER APPLICATIONS 



181 



lator. Connection is made through a duplex (double) wire 
attached to a plug, which can be screwed into an ordinary 
lamp-socket. 

It is better, however, to have a special socket for the 
iron, as the current used is often large enough to burn 
out the connection in an ordinary 
socket. 

The pressure forcing the current 
through the heating element pro- 
duces the heat, and as the current 
is turned on while using, the iron 
remains hot. 

If the iron does not get hot 
enough, it may be fixed by short- 
circuiting one turn of its heating 
element, thus letting through more 
current. If it gets too hot, another 
turn may be added. Why? 

217. Other Applications. — Along 
with the flat-iron come many other 
electrical heating appliances. Some 
of these are the toaster, curling 
iron, stove, coffee percolator, and 
soldering iron. Any, and all, of 
these can be used on A. C. or D. C, 
and can be bought for different voltages, although the 
standard voltage is 110. 

The amount of current taken by these appliances varies 
with the appliance. A toaster usually requires from 1 to 
3 amperes ; a curling iron from | to 1 ampere ; a stove 
from 3 to 10 amperes ; a percolator from 2 to 5 amperes ; 
and a soldering iron from 1 to 2 amperes. 




Figure 181. — Parts of an 
Electric Flat-iron. 

1. Cover and handle. 
2. Cast iron plate that 
fits over heating ele- 
ment. 3. Heating ele- 
ment. 4. Base on which 
heating element rests. 



182 



HEATING EFFECT OF CURRENT 




Figure 182. — An Electric Grill. 
Can be used for several methods of cooking. 



"\ 


IS 


]l 


1 *» #f I 


\ 






Jl^Ag/ 




v?'"y 




Figure 183. — Electric Coffee 
Percolator. 



Figure 1 84. — Electric Cook Stove. 



OTHER APPLICATIONS 



183 



Electrical heating appliances are coming more and more 
into common use, principally from the fact that they 
are very convenient and 
at the same time are 
so clean and sanitary. 
Even the electric cook 
stove is now quite com- 
mon. It has become so, 
largely because it does 
away with objectionable 
coal and gas fumes. 

Electric cars are com- 
monly heated by electric 
registers, and electric 
heaters are often used in 
homes, especially to heat 
small rooms, like bath- 
rooms. During weather 
which is too warm to 
require a furnace fire, 
and yet is too cold to 
keep the house comfort- 
able without a little heat, electric heaters leave the air 
purer than those which burn gas or oil. 

In buying any electrical appliance, care should be used 
to get a good one, as the extra cost at the beginning is soon 
saved in the saving of electrical energy to run it. 




Figure 185. — Electric Ironing Machine. 
Heated and Run by Electricity. 



CHAPTER XVII 

MOTION-PRODUCING EFFECT OF AN ELECTRIC 
CURRENT 



218. How Motion is Produced. — We saw in the case of 
a coil of wire revolved in a magnetic field that a current 
was produced in the coil. The reverse of this is also true. 
If a coil of wire is put into a magnetic field and a current is 

sent through the coil, 
it is made to revolve. 
With the aid of Figure 
186 we will show why 
it will revolve, and in 
which direction the 
motion will take place. 
Let the current go 
through the coil in the 
direction ABODE 
F. Then the coil be- 
comes a magnet with 
its north pole (N c ) at the top face of the coil, and its south 
pole (S c ) at the bottom face of the coil. 

Now, since like poles repel and unlike poles attract, the 
coil is made to revolve clockwise, or in the direction of the 
small arrow at E. Thus we see that the coil is made to 
turn and that the turning effect is due to attraction and re- 
pulsion of magnetic poles. 

184 




Figure 186. — How Motion is Produced 
by Electricity. 



THE GALVANOMETER 



185 



APPLICATION OF MOTION- PRODUCING EFFECT OF AN 
ELECTRIC CURRENT 

219. The Galvanometer. — The galvanometer is an in- 
strument used to detect an electrical current in a conductor. 
It consists of a coil of wire (C, Figure 187) suspended between 
the poles (X and S) of a permanent magnet by means of a 
phospor-bronze ribbon 
ending in a small spring 
at the bottom. 

The current to be de- 
tected is sent through 
the coil making it an 
electro-magnet. If the 
current passes down- 
ward, as the arrow in- 
dicates, the north pole 
of the coil is to the 
left of the coil. 

The permanent S-pole 
then attracts it, and 
the coil is made to turn 
as the arrows indicate. 

If it were not for the spring, the coil would turn until its 
north pole would be directly in front of the permanent 
S-pole, and would then stop. But the spring allows it to 
turn only so far as the strength of the poles forces it. Since 
the strength of the poles depends upon the current flowing 
in the coil, the deflection of the coil indicates not only that 
there is a current, but its relative strength. 

To make the reading of the deflection easy, a pointer is 
attached to the coil (or sometimes a mirror is used, so that 




Figure 187. — Wiring Diagram of a 

Galvanometer. 



186 MOTION-PRODUCING EFFECT OF CURRENT 





Figure 188. — Wiring Diagram Showing where Ammeter and 
Voltmeter are Placed. 

a ray of light may be deflected), showing the amount of 
deflection. 

220. The Ammeter. — The galvanometer detects current 
flowing, and its relative value, but does not give its amount 
in amperes. 




Figure 1 89. — Photograph of a Voltmeter with the Cover 

Removed. 



THE VOLTMETER 



187 



When the galvanometer has its scale graduated in amperes, 
it is called an ammeter. Its principle is just the same as the 
galvanometer, but reads directly in amperes. 

The resistance of the coil in an ammeter is wry low, so 
that it must always be placed in the line {A, Figure 188), 
and never across the line. 

221. The Voltmeter. — The voltmeter is also like the 
galvanometer, consisting, as it does, of permanent magnets 




Figure 190. — The Permanent Magnet, Coil, and Pointer of a 
D. C. Voltmeter. 



(D. C. meter) and a suspended coil. The scale of the volt- 
meter is graduated to read directly in volts. 



188 MOTION-PRODUCING EFFECT OF CURRENT 

The resistance of the voltmeter is made very high; so it 
should be placed across, not in, the line (V, Figure 188). 

This resistance is made up of the resistance of the mov- 
able coil of the instrument. When a high resistance is desired 
fine wire with a large number of turns is used, but when 
a low resistance is needed the coil is wound with a coarse 
wire with few turns. 

It is essential that you know how to connect a voltmeter 
and an ammeter correctly. Should you put the ammeter 




Figure 191. — The Movable Coil and Pointer of a Voltmeter. 

across the line, it will be burned out. Should you place the 
voltmeter in the line, it will shut off almost all the current. 

222. The Wattmeter. — The wattmeter is an instrument 
made to read the power used in a line. It consists of two 



THE WATTMETER 



189 




Figure 192. — A Volt-ammeter which can be used as either a 

Voltmeter or an Ammeter. 

The metal binding posts are ammeter connections, and the rubber 
ones are voltmeter connections. 

sets of coils. One set takes the place of the permanent 
magnets in the ammeter, voltmeter, and galvanometer, and the 
other coil is movable, as in the above instruments. 




Load 



Figure 193. — Wiring Diagram of a Wattmeter. 



190 MOTION-PRODUCING EFFECT OF CURRENT 

Since the wattmeter measures power, it must read in 
watts, or volts X amperes. 

It is so connected (Figure 193) that the current passes 
through the field coils, measuring the current; and the 
movable coil is connected across the line, measuring the volts. 
The deflection then reads 

Volts X Amperes = Watts. 

223. Meters for A. C. Electricity. — The meters here 
described are for D. C, although the wattmeter will work 
on either A. C. or D. C. But a special kind of ammeter 
and voltmeter must be made for A. C. They must have 
electro-magnets, instead of permanent magnets. 

224. D. C. Motors. — We have shown how a loop of wire 
with a current in it tends to revolve when placed in a mag- 
netic field. But its tendency is to revolve no farther than 
to bring the face of the coil which is a N-pole opposite the 
S-pole of the field magnet, and to remain in this position. 

Now, if the current is reversed in the coil, the face which 
was a N-pole becomes a S-pole, and vice-versa ; and the coil 
is made to revolve another half-turn. If the current is 
again reversed, the coil makes another half-turn ; and so on. 
Thus the coil is made to turn continuously by reversing the 
current in the loop every half -turn. 

You will remember that the alternating current generated 
in the loop of wire of the generator was made direct by 
means of a commutator. In the same way a direct current 
is made to reverse in the loop of wire in the motor. Thus 
by putting a commutator on the loop of wire the coil is 
made to turn continuously. Do not forget that the turning 
effect is due to the attraction of magnetic poles. 

The difference between a generator and a motor is this: 



THE WATT-HOUR METER 



191 



the generator is supplied with mechanical energy, and trans- 
forms it into electrical energy; while a motor is supplied 
with electrical energy, and transforms it back to mechanical 
energy. A direct current generator may be used also as a 
motor. 

225. The Watt-hour Meter. — The principle of the watt- 
hour meter is the same as the wattmeter, but instead of the 
movable coil being held 
in position by a spring 
it is allowed to turn 
around freely, as a 
motor. Geared to the 
movable coil are small 
hands which pass over 
dials, just as in the gas- 
meter. 

With one turn of the 
coil one watt-hour is 
registered on the dial; 
but this is such a small 
unit that it cannot be 
detected. One thousand 
turns make a kilowatt- 
hour, and this is indi- 
cated by 1 on the first 
dial. 

The reading of the watt-hr. meter is the same as the 
gas-meter (refer to gas-meter, § 69) . 

At the bottom of the meter is an aluminum disk revolving 
between permanent magnets. This disk acts as a brake, 
so that the coil revolves at a speed proportional to the watts 
used ; it also stops the meter when the current is turned off ; 




Figure 194. — A Direct Current Watt- 
hour Meter with Cover Removed. 



192 MOTION-PRODUCING EFFECT OF CURRENT 



otherwise the coil would 
coast and register watt- 
hours which were never 
used. 

Watch your meter at 
home speed up when lights 
are turned on and slow 
down when they are turned 
off. It should stop when 
all appliances are off; and 
if it does not, have it re- 
ported, as you are paying 
for electricity not used. 
Be sure that you can read 
your meter, and then check 
your light bills. 

226. The Starting-box.— 
The resistance of a motor is 

very small, usually not over \ ohm. If it were attached 

directly to the line, as is shown by Figure 196, the coils of 

the motor would be burned out. The reason for this is 

easily seen. If the voltage is 

110 volts and the resistance is "ov 

\ ohm, the current would be 

110 




Figure 195. — An Alternating 
Current Watt-hour Meter. 



^Mh 



= 220 amperes, which would 




Figure 196. — Wiring Diagram 
of a Motor Directly Across 
the Line. 



burn out the coils. 

In order to protect the motor 
when starting, a " starting-box " is used. This is made up 
of coils of resistance wire placed in a convenient box, so 
that the coils may be cut out of the circuit by merely 
moving a handle over to the right. (Figure 197.) 



C. E. M. F. 



193 




Figure 197. — Wiring 
Diagram of a Simple 
Starting-box. 



The first coil begins at notch No. 1 and ends at No. 2. 
The second coil starts at No. 2 and ends at No. 3, and so on. 
When the arm is on No. 1 notch the 
current must pass through all five coils. 
As the arm is moved to the right, coils 
are cut out. 

227. C. E. M. F. — It is easy to see 
why the starting-box keeps the current 
small, and thus protects the motor while 
the coils are all in the circuit ; but it is 
not so easy to see why the current does not get large when 
the coils are cut out. 

You will remember that we said that whenever lines of 
force are cut by a conductor an electric pressure is generated. 
Now, a motor, when running, has loops of wire (the arma- 
ture) turning in a mag- 
netic field (field), and 
thus an electric pressure 
is generated. This pres- 
sure is in the opposite 
direction to the applied 
pressure or E. M. F., and 
is hence called counter- 
E. M. F. or C. E. M. F. 
A motor, then, when 
running, generates a 
C. E. M. F. which 
opposes the applied 
E. M. F., thus neutraliz- 
ing part of it. On account of this, the coils of the starting- 
box may be cut out, as the C. E. M. F. holds the current 
down when the motor has gotten up to speed. 




Figure r 



— Photograph of a 3-point 
Starting-box. 



194 MOTION-PRODUCING EFFECT OF CURRENT 

Suppose the motor mentioned above generates 100 volts, 
C. E. M. F., when running at full speed, 

then = — = 20 amperes, the amount of current 

2 2 

the motor would take when running at full speed. 




Figure 199. — Photograph of a 4-point 
Starting-box. 



228. Series Motor. — There are three general classes of 
D. C. motors : Series, Shunt, and Compound. We shall dis- 
cuss only the first two. 




Figure 200. — Wiring Diagram 
of a Series Motor with 
Starting-box in the Circuit. 




Figure 201. — Wiring Diagram of 
a Shunt Motor with Starting- 
box Connections. 



SHUNT MOTOR 



195 



Figure 200 shows the connection for a series motor with 
starting-box in the circuit. 

The term series is used because the armature and field 
are connected in series. The starting-box is put in the line, 
in series with the arma- 
ture and field. 

The speed of the 
series motor is regulated 
by putting a resistance 
in series with the motor. 
To make the motor run 
fast, cut out resistance ; 
and to make it run 
slowly, put in resistance. 
Why? 

Series motors are used 
where the motor must 
start under load, as in 
the case of a street car 
or an elevator. Why? 

229. Shunt Motor.— 
The term shunt is used 
because the armature 
and field are placed in 
" shunt," or parallel. 
Figure 201 shows the connections of a shunt motor with 
starting-box attached. 

The current comes in at the switch, passes to the point 
on the starting-box marked " Line." From the point 
marked " A " a wire leads to the armature ; and from the 
point marked " F " a wire goes to the field. The other 
ends of the field and armature are connected together, 




Figure 202. — Vacuum Cleaner Driven 
by an Electric Motor. 



196 MOTION-PRODUCING EFFECT OF CURRENT 




Figure 203. — -An Electric Fan. 



and then attached to the 
other side of the line at 
the switch. 

Inside of the starting- 
box, a wire goes from 
the point marked 
" Line " to the arm. 
From the last notch goes 
a wire to the point 
marked "A," and from 
the first notch goes a 
wire to a small coil C, 
and then to the point 
" F." 

To start the motor, 
close the switch; then 

move the arm of the starting-box slowly to the right, 

allowing the motor to 

pick up speed. 

This cuts out the re- 
sistance in the armature 

circuit, making the arma- 
ture turn faster ; and at 

the same time it puts 

resistance into the field 

circuit, which also makes 

the armature turn faster. 

(Why?) The small coil 

acts as a magnet and 

holds the arm over 

when it is pushed far 

enough. 




Figure 204. — A Small Electric Motor 
Used to Drive a Sewing Machine. 



SHUNT MOTOR 



197 




Figure 205. 



An Electrical Motor Designed to Run a 
Washing Machine. 



The speed is regulated by putting a resistance into the 
field circuit. Putting in resist- 
ance makes the motor speed up. 
Taking out resistance makes it 
slow down. It may seem unrea- 
sonable at first that putting in 
resistance in series with the field 
of a shunt motor speeds it up, 
and taking out resistance slows it 
down. 

The reasons for these charac- 
teristics are readily understood, 
however, when it is remembered 
that the thing that does most to 
control the current through a 
motor is the C. E. M. F. which 
it generates. 




Figure 206. — An Electrical 
Motor Attached to a Wash- 
ing Machine. 



198 MOTION-PRODUCING EFFECT OF CURRENT 




Figure 207. — A Large A. C. Power Motor Disassembled to Show 
Different Parts. (Slip Ring Type.) 

The armature must turn fast enough to generate a 
C. E. M. F. almost equal to the applied E. M. F. If the 
field is weak the armature must burn fast, but if it is strong 




Figure 208. — Another Large A. C. Power Motor Disassembled. 
(Squirrel Cage Type.) 



SPECIFIC USES OF A. C. AXD D. C. MOTORS 199 

then the armature need only turn slowly, to generate this 
necessary C. E. M. F. 

Therefore, since adding resistance in series with the field 
makes the field weaker, it causes the motor to speed up, and 
since taking out resistance in series with the field makes the 
field stronger, it causes the motor to slow down. 

This motor is used where it can start without load, and 
can then have the load thrown on gradually, as in the case 
of motors in a machine-room. 

230. Small Motors. — If the motor is small enough, it 
may be put directly on the line, without a starting-box. In 
this case the armature is so light in weight that it can start 
to full speed before the coils have time to burn out. 

231. Specific Uses of A. C. and D. C. Motors in the 
Home. — Motors for either A. C. or D. C. circuits are often 
used for the following purposes : 

1. Electric fans. 4. Kitchen motors. 

2. Sewing machines. 5. Vacuum cleaners. 

3. Washing machines. 6. Hair driers. 

Name any other uses you know. 



CHAPTER XVIII 



INDUCTION 



232. Permanent Magnet in a Coil of Wire. — Induction 

is the producing of an electrical pressure (E. M. F.) by means 

of a conductor cutting magnetic lines of force. This is not a 

new idea, but is one which we have been using all through 

v / the subject of Electricity. 

We spoke of it when we 

studied the simple generator. 

In the simple generator 

the conductor moved and 

cut the lines of force, which 

remained stationary. This 

action may be reversed, — 

the conductor remaining 

stationary and the field 

moving, — and the result 

will be the same. 

Figure 209 shows a per- 
manent magnet (M) thrust into a coil of wire (C), the ends 
of the coil being connected through the galvanometer (G). 
When this is done, the galvanometer will deflect, showing 
that a current passes through the coil. The lines of force 
come out of a N-pole and go around and into a S-pole. 
When the magnet is thrust downward, these lines are cut 
by the wire in the coil. 

200 




Figure 209. — A Permanent Magnet 
Being Thrust into a Coil of Wire. 



AN ELECTRO-MAGNET IN A COIL OF WIRE 201 



If the magnet were pulled out, the lines of force would 
be cut in the opposite direction, and the galvanometer would 
deflect in the opposite direction, showing that the current is 
reversed. 

Then, to thrust a X-pole in and pull it out immediately 
produces an A. C. current in the coil. 

Just the reverse action takes place when a S-pole is thrust in 
and pulled out, since the lines of force are reversed. That 
is, to pull a S-pole out is the same as to thrust a X-pole in, 
and to thrust a S-pole in is the same as to pull a X-pole out. 

233. An Electro-magnet in a Coil of Wire. — Figure 209 
shows a coil of wire with a permanent magnet thrust into it. 
Figure 210 shows the 
same coil of wire, but 
instead of a permanent 
magnet an electro- 
magnet has been used. 
The effect is exactly 
the same as before. 

Xow, if instead of 
thrusting in and pull- 
ing out this electro- 
magnet, the core with 
the wire around it is 
placed inside the coil 

of wire, and the key (K) is pressed and released, the same 
effect is obtained. 

While the key is open, the core is not a magnet ; then 
when it is pressed, the core becomes a magnet, giving the 
same effect as thrusting a magnet in. Again, when the 
key is released, the core loses its magnetism, and the result 
is the same as when the magnet is pulled out. 




Figure 210. — An Electro- magnet in a 
Coil of Wire. 



202 



INDUCTION 



Thus we see that if two coils are placed so that one is 
inside the other, and a current is made in one, a current 
is induced in the other. Also, if a current is stopped in 
one, a current is induced in the other, in the opposite 
direction. 

The coil in which the current is made or stopped is called 
the primary, while the coil in which the current is induced 
is called the secondary. 

234. Mutual and Self-induction. — The above case is 
called mutual induction. It is the producing of a current in 
one wire by the effect of a current in another. 




Figure 211. — Induction Apparatus. 

Self-induction has to do with but one wire. 

It takes time and energy to start an automobile. The 
tendency of the automobile to hold back, or stay where it 
is, is called inertia. The tendency for a current not to flow 
when it is being started, and to keep on flowing when it is being 
stopped, is called self-induction. 

Self-induction always takes place when a current is 



THE INDUCTION COIL 



203 



changed (made larger or smaller) in a circuit. It acts in the 
opposite direction to the change. 

235. The Induction Coil. — The induction coil or " spark- 
coil," is used to increase the pressure in a D. C. circuit so 
that a spark will jump across a gap. 

The wiring diagram of an induction coil is shown in Figure 
212. 

A coil of heavy wire (p) is wound on a soft iron core, with 
a few turns. Around this is wound a coil of fine wire, with 
many turns. The coil of p 

heavy wire is called the 
primary, and is connected 
in series with a push 
button (P), a battery (B), 
and a vibrator (T ). The 
fine-wire coil is called the 
secondary, and ends at 
opposite sides of a spark 
gap. A condenser (C) is 
placed across the gap 
made by the vibrator. 

A condenser is a storage 
tank for electricity. It is usually made up of layers of 
tinfoil insulated from one another by mica or other insulat- 
ing material, alternate layers being connected together. 
Positive electricity flows in on one side, and negative on 
the other. The more leaves or layers, the more it will hold. 

In the primary of the induction coil the action is the same 
as in the door bell, the vibrator flying backward and for- 
ward, making and breaking the current. Whenever the 
current changes in the primary, a current is induced in the 
secondary by mutual induction. 




Figure 212. — Wiring Diagram of 
the Induction Coil. 



204 INDUCTION 

Since there are several times as many turns in the second- 
ary as there are in the primary, the voltage of the secondary 
will be just that many times as great as in the primary. 

To explain : Suppose the primary has 10 turns and the 
secondary 1000 turns, and that the primary produces a 
field of a certain strength. Now, for every turn on the 
primary there are -j%^-, or 100, turns on the secondary. 
Hence, the secondary cuts 100 times as many lines of force 
as the primary. Since the voltage depends upon the num- 
ber of lines cut per second, the voltage in the secondary 
will be 100 times that in the primary, or 

voltage cf secondary turns of secondary 
voltage of primary turns of primary 

Since there is self-induction wherever a current is started 
or stopped, the making and breaking of the primary circuit 
is not accomplished quickly. The condenser is put in over 
the gap to make this action take place more quickly, thus 
increasing the voltage of the spark. 

236. Uses of the Induction Coil. — The induction coil is 

| rs used in igniting the gas in gas engines. 

M^^g/Kk " ' ~~~~ -- 4.1t * s a ^ so use d for medical purposes. 

SRpp|^ri~- '- S 237. The Transformer. — The in- 

IliliB B *^lV^ duction coil was used on D. C, the 

I if ' i 1 ' vibrator changing the current in the 

' I ill I -I i ' primary. Now if A. C. is used, a 

J if ;! 1 il I ' vibrator need not be put in, but the 

?■« IS bJS primary may be wound about a soft 

tlJIBP J* I Z$\ iron without any mechanism to regu- 

PHI % late it. The alternation of the 

Figure 213.— A Low current takes the place of the make 

Voltage Transformer. and break of the induction coil. 



THE TRANSFORMER 



205 



Such an arrangement is called a transformer. It consists 
merely of two coils wound on a soft iron core. One coil is 
made of fine wire with many turns, while the other is made 
of heavy wire with few 
turns. 

As in the induction 
coil, the voltages of the 
coils depend upon the 
ratio of the number of 
turns. The coil which 
has the current put into 
it is called the primary, 
while the one in which 
the pressure is induced 
is called the secondary. 

The commercial trans- 
former has four coils ; 
two with fine wire, and 
two with coarse wire, 
wound on the same com- 
mon core of laminated 
soft iron. The ratio of 
turns in these coils is 
10 to 1. That is, for 
every turn on a coarse- 
wire coil there are 10 turns on a fine- wire coil. 

By connecting the coils in different combinations different 
voltages may be obtained. 

With a 110-volt primary line six voltages may be obtained 
with a commercial transformer — three by using the coarse- 
wire coils as primary, and three by using fine-wire coils as 
primary. 




Figure 214. — A High Tension 
(Voltage', Transformer. 



206 



INDUCTION 



P rimary 



NOV 



////.ru 



2 



Secondar y 



-, 2200 V 



Figure 215. — 110 Volts 
Transformed to 2200 Volts. 



P rimary 



IIOV 



7J7T 



IIOOV 



238. Coarse-wire Primary. — 1. If the primaries are 
connected in parallel, and the secondaries in series, the volt- 
age will be ^ X 110 = 2200. 
(Figure 215.) 

2. If the primaries are con- 
nected in parallel and the sec- 
ondaries in parallel, the voltage 
will be VX 110 = 1100. (Figure 
216.) 
3. If the primaries are connected in series and the 
secondaries in parallel, the voltage will be V X 110 = 550. 
(Figure 217.) 

239. Fine-wire Primary. — 1. If the primaries are con- 
nected in parallel and the second- 
aries in series, the voltage will be 
A X 110 = 22. (Figure 218.) 

2. If the primaries are con- 
nected in parallel and the second- 
aries in parallel, the voltage will 
be ^ X 110 = 11. (Figure 219.) 

3. If the primaries are connected in series and the 
secondaries in parallel, the voltage w r ill be ■£$ X 110 = 5 \. 
(Figure 220.) 

, , 240. Uses and Advantages of the 

Transformer. — First of all, you 
must remember that transformers 
can be used only on A. C. 

They are used for stepping the 
voltage up or down. Your house 
circuit is not in electrical connec- 
tion with the power station, but comes from a transformer 
near the house, where the voltage has been stepped down 



Figure 216.— 1 10 Volts 
Transformed to 1100 Volts. 



P rimary 



1 



rrnr. 



& 



I Secon dary 



Figure 217.— 110 Volts 
Transformed to 550 
Volts. . 



USES AND ADVANTAGES OF TRANSFORMER 207 



Secondary 



22V 



TTUVi 



Primary 



HOV 



Figure 218. — 110 Volts Trans- 
formed to 22 Volts. 



Se condary 



IIV 



mrm 



p; 



Prima ry 



MOV 



from 2300 volts to 110 volts. In fact, wherever power 
is to be delivered some distance it is sent out at high 
voltage, and then stepped down so that it can be used. 

The transformer has many 
advantages, but the four prin- 
cipal ones are these : 

1. It makes it possible to get 
any voltage you like from any 
voltage delivered. 

2. It saves cost of wire. Since 
power = E • I, if the power is sent out at a large volt- 
age, the current may be small, and since it is the current 

that heats a wire, the wire may 
be small when the current is 
small. 

3. It saves line drop, or fall of 
voltage. The fall of voltage along 
a line is the resistance of the line 
X the current flowing. We saw 
how the current could be made 

smaller with the transformer, and so line drop is cut down. 
4. It saves line loss. Line loss is power lost in the line, 

and is the line drop X current. Since the transformer makes 

it possible to reduce both the line 

drop and the current, it makes 

it possible to reduce the line loss. 
On account of the advantages 

just named nearly all transmis- 
sion lines are of high tension 

(voltage) . Being of high voltage, 

they are dangerous, and so are usually put up on strong 

towers, very well insulated, the wires themselves being bare. 



Figure 219.— 110 Volts 
Transformed to 1 1 Volts. 



S econdary 




Figure 220.— 1 10 Volts 
Transformed to 5h Volts. 



208 



INDUCTION 




241. The Three-phase System. — Heretofore we have 
always considered an electric circuit as having two lines, one 
line out and one line back. 

The modern system of delivery is 
what is called the " three-phase " 
system. It consists of three wires in- 
stead of two, and carries three times as 
much power as a two-line system. 

The generator for three-phase current 
is so arranged that the current goes out 
on one of the wires and comes back on 
the other two, or goes out on two and comes back on one. 

For example, at one instant the current is flowing out on 
line No. 1 (Figure 221), and at the same time is coming back 

Sub-Station 
Transformer 



Figure 221. — Wiring 
Diagram of a 3- 
phase Generator. 



Generator 



on poles 




Figure 222 



Wiring Diagram of a 3-phase 
City System. 



on No. 2 and No. 3 ; an instant later it will go out on No. 2, 
and come back on No. 1 and No. 3, etc. 

This is the system used in Cleveland, Ohio, by the Illu- 
minating Company. 



WIRIXG DIAGRAM OF HOUSE CIRCUIT 



209 



242. The Wiring Diagram of a City System. — Figure 222 
shows the general wiring diagram of a city using a 3-phase 
current. The elec- 

Q 







L <y 



tricity is generated 
at the generator (6) 
at 11,000 volts, and 
is sent out to the 
sub-stations (S) in 
conduits under 
ground. Here it 
runs through trans- 
formers and is 
stepped down to 
2300 volts. This is 
carried out on poles 
to the locality in 
which it is to be used. 
Here it is stepped down to 110 volts by transformers placed 
on the poles. This 110- volt line is carried into the houses. 
243. Wiring Diagram of House Circuit. — The current is 
brought into the house on two insulated wires at 110 volts. 




Figure 223. 



Wiring Diagram of a House 
Circuit. 




B B 

Figure 224. — Wiring Diagram of a Simple Telephone Circuit. 



210 



INDUCTION 




A city ordinance usually requires that all new wiring must 
enter the house at the basement. Just after it enters the 
house it passes through fuses. (Pi, Figure 
223.) Then it goes through the service 
switch (S) to the meter (M) ; then through 
another set of fuses (P 2 ) ; and then to the 
fixtures in the house ; all the appliances being 
put in parallel, across the line. 
244. The Telephone. — The telephone uses 

Figure 225. — A 
Portable Tele- 
phone Receiver 
and Trans- 
mitter. 

the principle of 
the transformer. 
Figure 224 shows 
a diagram of the 
simple Bell tele- 
phone. 

In the trans- 
mitter is a layer 
of powdered car- 
bon (0) between 
two plates ( P and 
P). By this ar- 
rangement an 
electric circuit is 
established, pass- 
ing through this 
carbon to a bat- 
tery (B), and through the primary of the transformer (T). 

The secondary circuit consists of the following parts, all 




Figure 226 — A Desk Telephone Switchboard 
such as is Used as a Local Switchboard by 
a Large Business Concern. 



THE TELEPHONE 211 

being put in series : (a) the secondary coil of the local trans- 
former, (b) the secondary coil of the transformer at the 
other station, (c) the coil of wire about the permanent 
magnet at the local station, (d) the similar coil about the 
permanent magnet at the other station, and (e) the connect- 
ing line wires. 

When the speaker talks into the transmitter, the little 
plate P alternately squeezes and releases the carbon, thus 
reducing and increasing its resistance. This causes the cur- 
rent in the primary to fluctuate. This induces an alternat- 
ing current in the secondary, which in turn strengthens and 
weakens the permanent horseshoe magnets. As these mag- 
nets are strengthened and weakened, they first pull, and 
then release, the steel plate (P 2 ) in the receiver, causing it to 
flip backward and forward. This plate (P 2 ) then reproduces 
the sound that enters the transmitter. 



CHAPTER XIX 
CHEMICAL RELATION OF AN ELECTRICAL CURRENT 

245. The Electrolytic Cell. — Sometimes liquids instead of 
solids are used as conductors of electricity. For instance, a 
salt solution will conduct electricity. When the current 
passes through a solution like this, a chemical change takes 
place which is quite different from what happens when a 
substance like mercury conducts electricity. 

The solution, with the points of contact, is called an 
electrolytic cell. 

246. Chemical Action in an Electrolytic Cell. — When a 
solution is made, part of its molecules break up into parts 
or ions, and are said to ionize. Before this can be under- 
stood a few terms must be learned. 

An atom is the smallest known part of an element which 
will enter into a chemical change. For example, a copper 
atom is the smallest known part of the element copper 
which will enter into a chemical change. We let the 
symbol Cu stand for it. 

A radical is a group of atoms acting as a single atom in 
a given chemical change. For example, in CuSO^ the SO4. 
is called a radical, and does not break up in a given chem- 
ical change. 

An ion is an atom or a radical, with an electrical charge. 
For example, a Cu atom with a charge of electricity is called 
a copper ion, and is written Cu + . Also, the radical SO4 

212 



THE ELECTROLYTIC CELL 



213 



with a charge of electricity becomes an ion, and is called a 
Sulphate ion and is written *S0 4 ~. Positive ions carry posi- 
tive charges, and negative ions carry negative charges. The 
same kind of atoms or radicals always carry the same kind 
of charge. 

Thus, when we say a solution ionizes, we mean it breaks 
up into atoms and radicals carrying electrical charges. 

When an electrical current passes through a solution, the 
positive ions are made to flow with the current, while the 
negative ions flow in the other direction. Also, more of 
the solution ionizes. This is the way a solution conducts the 
current. 

247. Parts of an Electrolytic Cell. — The parts of an 
electrolytic cell are (1) the solution, which is called the elec- 
trolyte; (2) the contact, or pole where the current comes in, 
called the anode; and (3) the contact, or pole where the 
current goes out, called the cathode. 

248. The Copper Sulphate (CuS0 4 ) Electrolytic Cell. — 
A solution of CuSo A , with a copper anode and any other 
conductor for a cathode, will 
make an electrolytic cell. 
(Figure 227.) The action is 
as follows : 

When the current is turned 
on, the CuSoi ionizes (some of 
it is already ionized) into Cu + 
and SOa~. The Cu + passes 
over to the cathode and gives 
up its charge, and places the 
Cu on the cathode. The £0 4 
passes over to the anode, unites with an atom of the copper 
plate, — with the aid of the positive charge coming through 



Cathode 



Any 
Conductor 




Figure 227. — A Copper Sul- 
phate Electrolytic Cell. 



214 



CHEMICAL RELATION OF CURRENT 



the wire, — and forms new CuS0 4 . As this action contin- 
ues, the cathode becomes plated with copper, and the 
anode is eaten away. 

This action can be expressed by the three following 

equations : 

CuSO* — ■>- Cu + + SOf 

Cu + — ->- Cu + ( + ) 

S0 A ~ + Cu + ( + ) — >- CuSOt 

249. The Sulphuric Acid (H 2 S0 4 ) Electrolytic Cell. — A 
solution of H2SO4 with a cathode and anode of platinum 

will form an electrolytic 
cell. (Figure 228.) 

The action is as follows : 
The H2SO4 ionizes into 
2H 2 + aaidSOf. The2H 2 + 
passes over to the cathode 
and there deposits its 
charge, the free hydrogen 
bubbling off as a gas. 
The SOf passes over to 
the anode, but cannot 
attack the platinum, so 
it unites with a molecule 
of water (H 2 0), with the 
aid of the positive charge 
( + ) coming through the wire, and forms a new mole- 
cule of H 2 SOt, the remaining oxygen bubbling off as a 
gas. As this action continues, the two plates remain the 
same, but the solution becomes concentrated, as H 2 is 
taken off in its two constituent gases. 

This action may be expressed by the three following 
equations : 




psggggg^ 



Figure 228. — ^A Sulphuric Acid 
Electrolytic Cell. 



ELECTRO-TYPING 215 

H 2 SO*— +2H 2 + + S0 A - 
H 2 + —^2H + ( + ) 
SO,' + H 2 + ( + ) — >- H,SO, + 

There are many different electrolytic cells but the action 
in all is similar to that in the two just studied. 

250. Electro-plating. — The electrolytic cell is used in 
plating. A solution containing a salt of the metal to be 
plated on the object is used as an electrolyte. The object to 
be plated is used as a cathode, and the anode is of the same 
material as the metal to be plated on the object. The 
action is exactly the same as in the case studied under 
the CuSOt electrolytic cell. 

Many precautions are required to make plating success- 
ful. The solution must be of just the right strength, the 
object to be plated must be perfectly clean, and the rate 
of plating, or the size of the plating current, must be just 
right. 

It is by this process that nearly all modern plating is done. 
Name some things that are silver-plated. Some that are 
nickel-plated, some that are gold-plated. 

251. Electro-typing. — Electro-typing is another of the 
useful things done by means of the electrolytic cell. All 
the cuts in books, magazines, and newspapers as well as the 
reading matter of most of our books are made by electro- 
typing. (The reading matter of most newspapers is not 
electro-typed.) 

If the thing to be electrotyped is a page of printed matter, 
the type is first set up. Then an impression is made in wax. 
This impression is next sprinkled with graphite to make it 
a smooth conducting surface. Then this form is used as the 
cathode in a plating cell. Copper about the thickness of 



216 CHEMICAL RELATION OF CURRENT 

paper is plated on the graphite surface. This is then backed 
with type-metal to make it strong, and the wax is melted 
off. This plate can then be used as often as desired, and is 
easily stored away. The type used at the beginning can 
be used over and over again. 



CHAPTER XX 
BATTERIES 

252. The Simple Voltaic Cell. — We have learned that 
an electrical pressure is generated whenever lines of force 
are cut by a conductor. Here are three other known ways 
by which an electrical pressure may be produced : 

1. By chemical action. 

2. By certain kinds of friction. 

3. By heating two metals in contact. 

If a glass jar has a solution of common salt put into it, 
and a zinc strip and copper strip be put into the solution 
and joined together by a conductor, an electrical current 
will now. The jar of salt water with its copper and zinc 
strips is called a voltaic cell, for it generates an electrical pres- 
sure. The pressure is set up by the chemical action which 
takes place in the cell. 

Care should be taken not to confuse the terms " voltaic 
cell " and " electrolytic cell." The latter is merely a con- 
ductor of electricity, while the former produces an electrical 
pressure. 

253. The H 2 S0 4 Voltaic Cell. — There are several kinds 
of voltaic cells. We just learned that salt water with 
copper and zinc strips for " electrodes " forms a voltaic cell. 
So, also, does dilute HoSO^ with copper and zinc electrodes. 

Let us note the chemical action that takes place in the 
H 2 S0 4 voltaic cell. (Figure 229.) 

217 



218 



BATTERIES 



As soon as the circuit is closed, the ionized H2SO4 sepa- 
rates, the H 2 going to the Cu electrode and giving up its 

charge, the 2 H being given 
off as a gas. The S0 4 goes 
to the Zn plate, receives the 
positive charge coming around 
the wire, and unites with the 
Zn to form ZnSO, (zinc sul- 
phate) . 

This action may be shown by 
the three following equations : 

H 2 S0 4 — >- H 2 + + SO,- 

H 2 + _^2#+( + ) 
SO,- + Zn + (+) — >- ZnSO* 




Figure 229. — A Sulphuric Acid 
Voltaic Cell. 



Thus we see that an elec- 
trical current is sent through 
the wire, that the H 2 SO± is used up, that ZnSOi is made in 
its place, and that the Zn strip is eaten up. 

254. Polarization. — It was seen above that hydrogen gas 
is given off at the copper plate. In all cells where this is 
done there is a tendency for these hydrogen bubbles to stick 
to the plate, and thus insulate it. This is called polarization. 

255. Open-circuit Cells. — Cells which polarize cannot 
be run for long periods, because the positive plate becomes 
insulated by the hydrogen. Therefore these cells are called 
" open-circuit cells," because the circuit on which they are 
placed must remain open most of the time and can be closed 
for only short periods. 

Name some uses of open-circuit cells. 

256. The Wet Salammoniac Cell. — An open-circuit cell 
may be made by placing a handful of ammonium chloride 



THE ADDWATER CELL 



219 



I 

Hi 



Zn 
NH.CI 

MnO, 



Figure 230. — Cross 
Section of a Simple 
Dry Cell. 



(NH4CI) in a quart jar filled with water, using a strip of 
carbon for a positive electrode and a zinc strip for a nega- 
tive electrode. This cell is often used 
for doorbells. 

257. The Dry Cell. — The dry cell 
has the same chemical action as the wet 
NH4CI cell, but it is constructed differ- 
ently, so that it may be handled much 
easier. 

Figure 230 shows a cross section of 
this cell. The outside, or case, is zinc, 
and acts as the negative electrode. The 
center portion (C) is a stick of carbon, which is the positive 
electrode. Packed in around this carbon stick is a paste of 
NH4CI and manganese dioxide (MnOi) . The NH^Cl is the 
active portion, and the manganese dioxide is put in to 
-oowet cap retard polarization. This is an 

open-circuit cell. 

The top shaded portion is tar, 
or wax, used to seal the cell so 
that the moisture will not dry out. 
This cell gives about 1.4 volts, 
and, when new, will give as high 
as 30 amperes on short circuit. 
Name some uses of the dry cell. 
258. The Addwater Cell. — 
The Addwater cell is an open- 
circuit cell, the construction of 
which is kept secret by the manu- 
facturers. Its advantage over the 
ordinary dry cell is the fact it will last longer, as it has a 
well to be filled with water, thus keeping it from drying out. 




Knur/ A/ut 
Acorn Head Post 
Sea/ 

Sand 

Sawdust 

Pu/pboard L imng 



Carbon E/ec/rode 



* Z/nc Can 



Pulpboord Bottom 

Figure 231. — Cross Section 
of a Commercial Dry Cell, 
as it is now Manufactured. 



220 



BATTERIES 



259. Closed-circuit Cells. — In the case of some voltaic 
cells there is no hydrogen given off in the form of a gas, and 
so these cells do not polarize. Keeping the circuit closed 
for a long period does not harm them, 
and they are called " closed-circuit 
cells." 

Name some uses for closed-circuit 
cells. 

260. The Gravity Cell. — The gravity 
cell consists of two solutions placed in a 
glass jar with copper and zinc electrodes. 
These two solutions are concentrated 
CuSO* and dilute ZnSO, (5-1) . The CuSO, 
is placed in the bottom, and the ZnSOi on 
top. They keep these relative positions 
on account of their difference in density, 
hence the name " gravity cell." 

The copper plate is placed in the C11SO4, 

and the zinc plate, or " crowfoot," is hung 

in the ZnS0 4 . The circuit must be kept 

closed, or the two liquids will diffuse, thus 

These cells are used on telegraph lines. 

261. The Daniell Cell. — The Daniell cell is similar to the 
gravity cell, except that the ZnSO^ is placed in a clay porous 
cup so that the cell may be handled without danger of mix- 
ing the solutions. The action is exactly the same as in the 
gravity cell. 

262. Secondary or Storage-cells. — The voltaic cells we 
have been studying are capable of giving an electrical pres- 
sure as soon as they are set up, and are therefore called 
primary cells. It has been found that cells may be made 
which will not at first give an electrical pressure, but which 




Figure 232. — The 
Addwater Cell, 
which is a Special 
Kind of Dry 
Cell. 

spoiling the cell. 



THE LEAD WET STORAGE-CELL 



221 



will do so if " charged." These cells are called " second- 
ary cells " or " storage-cells." 

263. The Lead Wet Storage-cell. — A storage-cell may 
be made by using two lead plates for electrodes and dilute 
HzSOi for an electrolyte. 
(Figure 233.) 

When first set up, this 
cell will not give a pres- 
sure, but if a D. C. current 
is allowed to flow through 
it for a time it is said to 
become " charged," and 
will then give an electrical 
pressure. 




||pb; §|i§llli P& % 






The charging current 



Figure 233. — A Diagram of a Wet 
Lead Storage Battery. 

causes a chemical action 

to take place within the cell, thus storing up chemical 
energy. No electricity is stored in the cell. Then, when 
the cell is used to give 
pressure, the current 
flows in the opposite di- 
rection, at the expense of 
the chemical energy stored 



■ — sMA/W^- 



Source of 
Pressure 



Figure 234. — Wiring Dia- 
gram of a Storage Battery 
Charging Circuit. 




Figure 235. — A Commercial Lead 
Storage Battery. 



in it. When this energy is exhausted, the cell must be 
recharged. 



222 BATTERIES 

To charge the cell, a D. C. must be used, and the + pole 
of the charging circuit must be connected to the + pole of 
the cell. (Figure 234.) 

If A. C. is used, it must first be rectified, that is, changed 
into D. C. by a motor generator, a rotary convertor, or a 
mercury vapor lamp. 

The lead storage-cell is easily injured, so a few precau- 
tions may be appropriately named : 

1. D. C. current must be used for charging. 

2. Do not overcharge. 

3. Do not short circuit. 

4. Do not charge too fast. 

5. Do not let it remain uncharged. 

6. Keep it filled with pure water. 

The lead storage battery is used for many things. Some 
of these uses are : 

1. To run electric motor cars. 

2. To start motors and to light cars. 

3. To light houses in the country. 

4. For plating. 

The lead storage-cell gives about 2 volts per cell, regardless 
of the size of the cell. 

264. The Dry Lead Storage-cell. — There has just re- 
cently been put on the market a dry lead storage-cell 
(Figure 236), but as yet, its success 
has not been shown. It may, or may 
not, be good. Its principle is ex- 
actly the same as the wet lead cell, 
but instead of the acid being in a 
free state, it is absorbed by a com- 

Figure 236. — Diagram of j ,-, e tl j ,, u 

a Dry Lead Storage P ound > thus formm g a dr - v cel1 - 
Battery. The electrodes are lead plates wound 




THE EDISON STORAGE-CELL 



223 



in concentric spirals, thus giving a large active area. The ab- 
sorbing compound is pressed in between the plates with such 
force that the active material on the plates cannot come out. 

If this cell proves to be good, it will be a great step in 
storage battery construction, for free acid is a dangerous 
thing to handle. 

265. The Edi- 
son Storage-cell. 
— Thomas A. Edi- 
son has had an 



OLAND CAP 




NEGATIVE GRII 



CIN INSULATOR 



SiOE INSULATOR- 



IDE ROD INSULATOR 



Figure 238. — The 
Positive and Nega- 
tive Plates of an 
Edison Cell. 




SUSPENSION BOSS 



Figure 237. — Dissected View of an Edison 
Storage Battery Cell. 



altogether different storage-cell on the market for some 
time. This cell has potassium hydroxide (KOH) for an 
electrolyte, and patented nickel and steel electrodes. The 
container is a pressed-steel box, so that it is almost in- 
destructible. The Edison cell does not need the care 
that a lead cell does, and can be subjected to much more 



224 BATTERIES 

rough handling, without injury. A short circuit does not 
permanently harm it, if it is immediately recharged. 




Figure 239. — A Wooden Tray Containing 
5 Edison Cells. 

The voltage of the Edison storage-cell is lower than that 
of the lead cell, it being about 1.5 volts; and its efficiency 
runs lower than the lead cells. 



STATIC ELECTRICITY 

266. Static Electricity. — Till now we have been study- 
ing about dynamic or current electricity. But there is 
another kind called static electricity. 

There are many applications of this form of electricity, 
such as lightning, wireless telegraphy, and medical uses. 
When we scuff across a thick rug in a cold room and then 
touch a metal door-knob or gas-fixture, we get a slight shock 
due to static electricity. 

Although the applications of static electricity are spectac- 
ular and interesting, it has not the widespread practical 



STATIC ELECTRICITY 



225 




Figure 240. — An Actual Photograph of a Stroke of Lightning 
Taken on the Shore of Lake Michigan. 

value of current electricity. For this reason a complete 
treatment of it is not embodied in this book. 






Review Problems 

1. Discuss the field about a magnet. 

2. Distinguish between a magnetized piece of iron and one which 
is not magnetized. 

3. Why is magnetism studied before electricity ? 

4. How may an electrical pressure be generated? What deter- 
mines its amount and its direction? 

5. Discuss pressure, current, and resistance. 

6. Distinguish between A. C. and D. C. 

7. How is an A. C. made D. C. ? 

8. Describe the space about a wire carrying a current. 

9. What determines the poles of an electro-magnet? 

10. Name ten applications of the electro-magnet. 

11. How does electricity produce heat? 



226 BATTERIES 

12. Name five electrical quantities to be measured, the unit used for 
each, and the letter used to denote each. 

13. If a door bell has 180 ohms resistance, what current will it take 
if 6 volts are applied to it? 

14. What is the cost of running a motor for 2 hours, if it takes 3 
amperes on 110 volts, the cost of electricity being 9£ per Kw.-hr. ? 

15. How long would a starting-battery last if it contained 600 watt- 
hours and gave a pressure of 6 volts at a 300 -ampere discharging rate? 

16. Compare the cost of running four 25-watt lamps to that of three 
40-watt lamps. 

17. How much would you save on your electricity bill if you had an 
attachment like the " dim-a-lite," which would throw in an additional 
100 ohms to the 340 ohms if the lamp were to burn 8 hours on a 110- 
volt circuit, and cost 9j£ per Kw.-hr.? 

18. In problem 17 would the lamp be as bright with the extra 100 
ohms in the circuit ? 

19. What heats an electrical flat-iron ? 

20. How does electricity produce motion? 

21. Explain how the ammeter measures current. 

22. Show where a voltmeter and an ammeter should go in a circuit. 

23. What is the difference between A. C. and D. C. meters? 

24. Discuss the essential parts of a watt-hour meter. 

25. What is C. E. M. F.? 

26. Tell briefly the difference between a series and a shunt motor. 

27. What is induction ? 

28. Discuss mutual- and self-induction. 

29. How could you get 6 volts from a 120-volt A. C. line? 

30. If the two coils of a transformer have their turns in the ratio of 
3 and 24, what voltages could you get from a 110- volt A. C. line? 

31. What is the advantage of the 3-phase system? 

32. Discuss the wiring diagram of a house. 

33. What is the difference between an electrolytic cell and a voltaic 
cell? 

34. Explain how silverware is plated. 

35. Why is a dry-cell called an " open-circuit cell "? 

36. Give some applications of static electricity. 



CHAPTER XXI 
MECHANICS OF SOLIDS 

267. Units of Measurement. — The things with which 
physics deals are very definite, and so require definite units 
to measure them. For example, the houses we live in are of 
definite sizes, the food we eat has a certain weight, and you 
go to class for a definite length of time. All these quantities 
are definite, and in order to express them we must have 
definite units. 

This is not a new thing, for we have been using units all 
through this course, but it is advisable to study them for 
their own sake. 

268. The English System. — There are two great sys- 
tems of measurement — the English and the Metric. There 
is no necessity for two systems, but we have them, and 
people will continue to use both for many years to come. 

There are other things to be measured, but the three 
principal ones are space, mass (incorrectly called weight), and 
time. 

Under space, come length, area, and volume. The English 
unit of length is the foot. Other units are derived from 
this ; the yard = 3 ft. ; the inch = T V ft. ; the mile = 5280 ft. 

The unit foot is made definite by the fact that the original 
is kept in London. Copies of it are made and used as 
standards of measurement. Our standard is kept at 
Washington. 

227 



228 



MECHANICS OF SOLIDS 




Figure 241. — A 
Cubic Foot. 



The units of area and volume are derived from the units 
of length. Thus the square foot is an area which is one foot 
on a side ; the cubic foot is a cube which 
is one foot on each edge. (Figure 241.) 

Other units, such as square yard, cubic 
yard, square inch, cubic inch, etc., have 
similar meanings. 

The unit of mass is the pound (lb.), 
and it denotes a certain amount of 
matter determined by a standard kept 
in the same way as the standard foot. Other units are 
derived from it, such as the ounce (oz.) = jg lb. ; the ton 
(T.) = 2000 lb. ; etc. 

The unit of time is 
the second (sec.) ; it is 
based on the time it 
takes the earth to 

make one rotation on its axis. The second is g 6lo"o °f that 
time. The other units derived from it are the minute 
(min.) = 60 sec. ; the hour (hr.) = 60 min. ; the day = 24 
hr. ; the year = 365 1 days. 




Figure 242. — -The Standard Meter. 




Figure 243. — United States National Prototype Meter Bar, 
Bureau of Standards, Washington, D. C 






THE TWO SYSTEMS COMPARED 229 

269. The Metric System. — The same quantities can be 
measured in the metric system, but the units are different. 
The unit of length is the meter (m.) ; and it is defined as the 
distance between two scratches made on a platinum bar 
kept at Paris. (Figure 242.) 

Table of Lengths 

10 millimeters (mm.) = 1 centimeter (cm.) 
100 cm. = 1 meter (m.) 

1000 m. =1 kilometer (km.) 

The metric unit of mass is the gram (gm.), and it is yoVo 
part of a piece of brass kept in Paris along with the standard 
meter. This piece of brass was so made that it has the 
same mass as 1000 c.c. of pure water at 4° C. That makes 
the gram equal to the mass of 1 c.c. of pure water at 4° C. 

Other units are given in the table. 

Table of Masses 

1000 milligrams (mg.) = 1 gram (gm.) 
1000 gm. = 1 kilogram (kg.) 

The metric unit of time is the second. It is identical with 
that of the English unit. 

270. The Two Systems Compared. — Just a glance at 
the two systems is sufficient to show that the metric is much 
the simpler. 

All the derived units in the metric system are multiples 
of ten. For example, 10 mm. = 1 cm., 100 cm = 1 m., 
1000 m. = 1 km., etc. This makes it easy to remember and, 
at the same time, easy to change from one unit to another. 
All that is necessary is to move the decimal point either to 
the right or left. For example : 



230 



MECHANICS OF SOLIDS 



1.273 m. = 127.3 cm. 
467.8 cm. = 4.678 m. 
3.642 kg. = 3642 gm. 

In the English system this is not true. There is no 
regularity whatever. This makes it hard to change from 
one unit to another. For example : 

15 ft. = 15 X 12 = 180 in. 
231 in. = W 1 

3 lb. = 3 X 16 
90 oz. = ff 

271. Relation between the Two Systems. — So long as 
there are two systems in use, we shall at times be obliged 
to change readings in one to readings in the other. For this 
reason we need a table of equivalents. The fact that the 
two systems are entirely independent makes these equiva- 
lents irregular and burdensome. 



19^ ft. 
48 oz. 
5flb. 





Table of Equivalents 


ENGLISH 


METRIC 


1 in. . . . 


2.54 cm. 


1 lb. . . . 


453.6 gm. 


1 sec. . . 


1 sec. 


1 sq. in. 


6.452 sq. cm. 


1 cu. in. 


..... 16.39 c.c. 


1 liquid qt. 


.945 liter (liquid unit) 



Using this table we can change from any reading in one 
system to the corresponding readings in the other system. 

272. Force. — Besides space, mass, and time there are 
many other physical quantities which have to be measured. 
One of these is force. 

Force is a push, or a pull, on an object, that tends to make 
the object move. The force may, or may not, make the object 
move, but it always tends to do so. For example, you can 



UNITS OF WORK 231 

pull on a chair and make it slide on the floor. Again, you 
can pull or push on the corner of a house, and it will not 
move, but there is a tendency to move, and if the push or 
pull were large enough, it would move. These are examples 
of force. 

273. Units of Force. — Force is measured in both the 
English and metric systems. 

The unit most used in the English system is the pound. 
You will notice that this is the same name as that given to 
the unit of mass, but the idea is different. 

A pound mass is a certain amount of matter. A pound 
force is the pull of the earth on a pound mass at sea level. 

The unit most used in the metric system is the gram. 
Again, this is the same name as that given to the unit of 
mass, and, as in the English system, it represents the pull of 
the earth on a gram mass at sea level. 

274. Work. — When a force produces motion, it is said 
to do work. Work is a definite physical quantity and can 
be measured. When you pull on a chair, and it slides on the 
floor, you do work ; but if you do not pull hard enough to 
make it slide or move, there is no work done. 

Work is the result of a force acting against a resistance and 
moving it. The amount of work is measured by the force 
multiplied by the distance the force moves. 

Work = Force X Distance. 

It will be seen that if the object is not moved, no work 
will be done ; or, if the body be moving without any force 
applied, no work is done. 

275. Units of Work. — The unit of work in the English 
system is the foot-pound, and in the metric system it is the 
gram-centimeter. 



232 MECHANICS OF SOLIDS 

A foot-pound is the work done when a pound force acts 
through a distance of one foot. 

If you were to pull a chair on the floor a distance of 3 
ft. and it took a force of 5 lb., the work done would be 

3X5 = 15 ft. lb. 

To find the work done, multiply the force by the distance 
it moves. 



CHAPTER XXII 



MACHINES 



276. Machines. — A machine is a mechanical apparatus 
ivhich either transforms or transfers energy. There are six 
simple machines. They are lever, wheel and axle, inclined 
plane, pulley, screw, and wedge. 

All other machines are composed of a combination of one 
or more of these six. For example, a sewing machine has a 
combination of the lever, pulley, and screw. Even the most 
complicated machine, such as the modern printing-press, is 
made of groups of the six simple machines. 

277. The Lever. — The lever consists of a rigid bar (B) 
Figure 244, a weight (IV), a force (F), and a pivot (P). W 
represents the force 
overcome, which is 
often the weight of 
an object being lifted ; 
F represents the force 
applied; while P is the 
point about which the 
bar turns. 

The distance (a) from the force to the pivot is called 
the force-arm. The distance (6) from the weight to the pivot 
is called the weight-arm. The product of the force and 
the force-arm is the force moment (F a), and the product of 
the weight and weight-arm is the weight moment (W b). 

233 




Figure 244. — The Lever. 



234 



MACHINES 




Figure 245. — ■ First Class Lever. 



The law of the lever is that the force moment equals the 
weight moment, or F a = W b. 

278. Classes of Levers. — Levers are divided into three 
classes, according to the relative positions of the force, the 

weight, and the pivot. 
The first class has 
the weight and the 
force on the ends and 
the pivot in the 
middle. (Figure 245.) 
The second class 
has the force and 
the pivot on the 
ends and the weight in the middle. (Figure 246.) 

The third class has the weight and the pivot on the ends 
and the force in the 
middle. (Figure 247.) 

279. Mechanical 
Advantage. — In dis- 
cussing a machine, 
the term mechanical 
advantage is used. 
Every machine has a mechanical advantage, and this is 
found by dividing the iveight by the force, or by finding an 

equal ratio. Thus it 
has a definite mean- 
ing, and is defined as 

W 
the fraction — • 
F 

In the case of the 
Therefore to find the 



iW 



Figure 246. — Second Class Lever. 




Figure 247. — Third Class Lever. 



lever - = -— 



w 

F 



(Figure 244.) 



APPLICATIONS OF THE LEVER 



235 



mechanical advantage of a lever, divide the force-arm by 

the weight-arm, or 

it 7 • ; j j. Force-arm 

Mechanical advantage = . 

Weight-arm 

280. Efficiency. — Another term used in discussing a 

machine is efficiency. This term also has a definite meaning, 

, . , n , ,, . ,. work-out 

and is denned as the traction — — • 

work-in 

Xo machine will do work of its own accord. Work must 
first be put into it, and then it will do work, giving a cer- 
tain amount out. The 
work-in is the work put 
into the machine. The 
work-out is the work 
that the machine gives 
out when operated. 

A machine never gives 
out as much work as is 
put into it, because some 
of the work is always 
lost in the machine, 
overcoming friction. Therefore the efficiency of a machine 
is always less than 100 per cent. 

In the case of a lever there is usually very little friction 
and so the efficiency is usually from 95 per cent to 99.9 per 
cent. 

281. Applications of the Lever. — There are many appli- 
cations' of the lever, but one that needs especial mention is 
the balance used for weighing objects. (Figure 249.) 

The balance consists of a beam (B) supported on a knife- 
edge (K). At each end of the beam is hung a scale pan (S) . 
These are also supported on knife-edges. A pointer (P) 




Figure 248. — Ball Bearings Reduce 
Friction and Increase the Efficiency. 



236 



MACHINES 





is attached to the beam 
to show when a balance 
of the weights is ob- 
tained. 

To make a weighing, 
the object to be weighed 
is placed in the left-hand 
pan and is the W of the 
lever. Standard weights 
are placed in the right- 
hand pan, so that a 
balance is obtained. 

The best method to 
get a balance is to start 
with the largest weight. 
If it is too small, add 
the next one, and so on. 
If it is too large, take it 
off and use the next smallest. Repeat this operation until 
a balance is obtained, that is, until the pointer will swing 
the same distance on one side as 
on the other. 

The balance is a lever of the first 
class. Other examples are shown in 
Figures 250, 251, 252. 

Figures 253, 254, 255 show applica- 
tions of the second class lever. 

Figures 256, 257, 258 show applica- 
tions of the third class lever. 

Make a simple drawing and 
classify the levers in the following 
examples. 



Figure 249. — The Weighing Balance 
is a Lever. 




Figure 250. — The Can 
Opener Used as a First ( 
Class Lever. 



APPLICATIONS OF THE LEVER 



237 




Figure 251. — The Tack 
Puller Used as a 
First Class Lever. 




Figure 253. — A Can 
Opener Used as a 
Second Class Lever. 




Figure 252. — Scissors Illustrate 
a First Class Lever. 




I I I 1VI 

Figure 254. — A Potato Ricer 
Used as a Second Class Lever. 




Figure 255. — A Nut Cracker is 
a Second Class Lever. 





OP 



I 

Figure 256. — Grass Cutters or F 

Sheep Shears Illustrate Third Figure 257. — The Sugar Tongs 
Class Lever. is a Third Class Lever. 



238 



MACHINES 



1. Wire pliers 

2. Pitcher pump 

3. Lemon squeezer 

4. Spoon 

5. Knife 

6. Fork 

7. Claw hammer pulling a nail 



8. 


Oar of rowboat 


9. 


Paddle of canoe 


10. 


The human arm 


11. 


Wheelbarrow 


12. 


See-saw 


13. 


Spring-board 


14. 


Shovel 




>p 



Name five other applica- 
tions of the lever, and 
classify them. 

282. Wheel and Axle. — 
The wheel and axle is an- 
other simple machine very 
similar in action to the lever. 

It consists of a wheel and 
an axle rigidly fastened to- 
gether. (Figure 259.) The 
force (F) acts on a rope 
wound around the wheel, 




Figure 258. — A Broom Used as a 
Third Class Lever. 



Figure 259. — The 
Wheel and Axle. 



INCLINED PLANE 



239 



and the weight (IF) is hung on a rope wound in the opposite 
direction on the axle. 

When the force moves down, the weight moves up. The 
action is the same as in the lever. 
The radius (R) of the wheel acts as 
the force-arm, and the radius (r) of 
the axle acts as the weight-arm. 

The mechanical advantage of the 
W 



wheel and axle 
lever, — • 



is -or, 



as in 



the 




Figure 260. — Another 
Form of the Wheel and 
Axle. 



The efficiency of this machine is 
less than that of the lever, ranging 
from 60 per cent to 99 per cent. 
The efficiency depends upon the 

bearings of the machine and upon the flexibility of the cord. 
Sometimes a crank is used instead of the wheel. (Figure 
260.) This does not change the action. 

283. Applications of Wheel and Axle. — The windlass 
used in removing dirt from wells or manholes in the street is 
an application of the wheel and axle. (Figure 261.) 

Another application of the wheel and axle is the device 

used for raising awnings. 
(Figure 262.) 

Name and draw two other 
applications of the wheel and 
axle. 

284. Inclined Plane.— The 
inclined plane consists of a 
plane set at an angle to the 
horizon. (Figure 263.) The 




Figure 261. — The Windlass Is a 
Wheel and Axle. 



240 



MACHINES 



weight (W) always acts downward, and the force CF) acts 
along the plane. The vertical distance (h) is called the 

height of the plane, while the 



distance along the plane (L) 
is called the length of the plane. 

The force (F) must move 
the length of the plane (L) 
in order to raise the weight 
(W) the height (h). 

The mechanical advantage 

W 

of the inclined plane 




is 



or 



— — • It will be seen from 



Figure 262. — A Wheel and Axle 
Is Often Used to Lift Awnings. 



this that the more nearly the 

plane comes to the horizontal, 

the greater will be the me- 

Then, in order to lift a large weight, 



chanical advantage, 
use a long plane. 

285. Applications of Inclined Plane. — There are many 
applications of the inclined plane. Figure 264 shows an in- 
clined plane used for loading a piano into a truck. A heavy 
plank is used for the plane and the height of the truck is the 
height of the plane. 
By this means one or 
two men can push 
the piano into the 
truck. 

Another applica- 
tion of the inclined plane is the rolling stairway. (Figure 
265.) This is often used in large department stores instead 
of elevators. A person wishing to go from one floor to 




Figure 263. — The Inclined Plane. 



PULLEY 



241 




another steps on the moving 
stairway and is carried up, or 
down, according to the direction 
in which the stairway moves. 
Usually there are two of these side 
by side, one going up, and the 
other down. 

Graded roads are excellent ex- 
amples of inclined planes. 

286. Pulley. — There are two types of pulleys 
266 and Figure 267.) 

Figure 266 shows two pulleys belted together. The 
one which supplies the power is called the driver, and 

the other the driven. 



Figure 264. — An Inclined 
Plane Used to Load a 
Piano into a Truck. 



(Figure 




Figure 265. — A Moving Stairway Is an 
Inclined Plane. 

The larger the driven pulley is, 
the greater the mechanical ad- 
vantage. 





Figure 266. — Two Pulleys Belted 
Together. 



6 

w 

Figure 267. — An- 
other Type of 
Pulley. 



242 



MACHINES 



Tne mechanical advantage 






radius of driven _ R 
radius of driver r 
Figure 267 shows the other type of pulley, often called a 
block. A block consists of one or more pulleys or sheaves 

fastened side by side, or 
one above the other, so 
that they are free to turn. 
Two blocks are used 
to lift a weight. One 
block is made fast, and 
the weight is attached 
to the other one. A 
rope or chain is threaded 
through the blocks, as 
shown in the figure. 

The mechanical ad- 
vantage is equal to the 
number of strands sup- 
porting the weight. 

From the figure it will 
be seen that if the weight 
be lifted 1 foot, there are 
six strands to be short- 
ened 1 foot. This allows 
the force (F) to move 6 
feet while the weight 
moves 1 foot. Thus the 
mechanical advantage is 
six. 

287. Applications of the Pulley. — A familiar example of 
the first type of pulley is the sewing machine. (Figure 269.) 
Here the large wheel is the driver, and the small wheel is the 




Figure 268. 



-A Laboratory Set of 
Pulleys. 



APPLICATIONS OF THE PULLEY 



243 






driven. This arrangement makes it harder to turn, but a 
greater speed can be obtained. 

The revolutions per minute (R. P. M.) of tic o pulleys belted 
together are inversely as their diameters. This means that the 
large pulley runs sloicly while the small 
one runs fast . 

Problem: If a driver is 2 ft. in diameter, 
and makes 500 R. P. M., what is the speed of 
the driven, which is \ ft. in diameter? 




Figure 269. — The 
Pulley as Used in 
the Sewing Machine. 



The second type of pulley is often C 
used in lifting safes or other heavy 
objects. (Figure 270.) A gin pole is 
placed in the window above, and the 
upper block is fastened to this. By pulling on the free end 
of the rope the safe is raised to the open window. From 

here it is swung inside. 

Elevators are usually lifted up 

and let down by means of this 

type of pulley. 




Figure 270. — A Set of Pul- 
leys Used to Lift Heavy 
Objects to the Upper 
Stories of High Buildings. 




Figure 271. — A Jack Screw. 



244 



MACHINES 




Figure 272. — A Wedge. 



288. Screw and Wedge. — The screw and the wedge are 
both very much the same as the inclined plane. As is shown 
by Figure 271 , the screw is merely a spiral inclined plane which 

is made to move 
under the weight, 
thus forcing the 
weight to move. 

Likewise Figure 

272 shows that the 

wedge is a double 

inclined plane, made 

to move under the 

weight, causing the 

latter to move. 

The pitch of a screw is the number of threads per inch, and 

the distance from one thread to the next is called the lead 

(L). The mechanical advantage is the circumference of the 

circle that the force moves divided by the lead, or 

Mechanical advantage = — - — 

The mechanical advantage of the wedge is the length of the 
tvedge (L) divided by the thickness of the wedge (h) } or 

Mechanical advantage = — 

h 

The efficiency of the screw and the wedge is small, because 
there is always much friction. 

289. Application of the Screw and Wedge. — The use of 
the screw is common, and many illustrations could be named. 
A few are the piano stool (Figure 273), the ordinary wood 
screw (Figure 274), and the bolt and nut (Figure 275). 

The wedge is not in such common use, but many examples 



POWER 



245 



can be found. Figure 276 
shows a hatchet used as a 
wedge to split kindling. 

290. Power. — Power is 
the time rate of doing work. 
It is very often confused 
with the term work; but it 
is different, for it involves 
the idea of time, while work 
does not. 

A boy could carry a thou- 
sand bricks up a ladder 10 ft. 
high as well as a man, but it 
would take him longer. 

The amount of work done 
by the boy and man would 

be the same, but the rate at which the man would do the 
work would be greater ; so we say 'he has the more power. 

The units of power are the foot-pound per second, and the 
gram-centimeter per second. These units are so small that 
larger units are commonly used. The horsepower is the one 
most common in this country. A horsepower is the power 
that will do 33000 foot-pounds of work per minute. 

To find the horsepower delivered in any case, find the 
__ work in foot-pounds done per minute, 

r7 and divide by 33000 ; thus : 




Figure 273. — The Piano Stool Is 
an Application of the Screw. 



\r 



Figure 274. — The 
Wood Screw. 



Figure 275. — The Bolt and Nut Is 
an Application of the Screw. 



246 



MACHINES 



If a girl weighs 120 pounds and climbs the stairs from one floor to the 
next, a distance of 15 ft., in 30 seconds, she does 120 X 15 = 1800 ft.- 
lb. in .5 min. (30 sec.) or 



1800 

.5 

3600 

33000 



3600 ft.-lb. per min. 

0.109+ horsepower. 



6^ 
55 




291. Power Delivered by Pulleys. — It is often desirable 
to know the power necessary to run certain appliances in the 

home, such, for example, 
as the sewing-machine, 
the vacuum cleaner, the 
washing-machine, food 
chopper, bread mixer, 
etc. Most of these are 
either run by pulleys 
driven by belts or by 
gears, so the method for 
finding the horsepower is 
the same. 

Let us compute the horsepower for a sewing machine as an 
example. 

Suppose the small 3-in. wheel of the sewing machine must make 
500 R. P. M., and that the belt has an effective pull of 2 lb. What is 
the horsepower necessary to run it ? 
Method : 

3_ 
12 



Figure 276. — The Hatchet Used in 
Splitting Kindling Is an Application 
of the Wedge. 



3 inches 



= .25 ft. 



.25 X 3.1416 = .7854 ft., cir. of wheel 

.7854 X 500 = 392.7 ft., distance the belt moves in 1 min. 

392.7 X 2 = 785.4 ft.-lb. per min. 

785.4 



33000 



.0238, horsepower required. 



PROBLEMS 247 

What horsepower is necessary to run a food chopper that requires a 
force of 10 lb. on the end of a 1-ft. crank making 60 R. P. M. ? 
Method : 

2 ft. = diameter of circle 
2 X 3.1416 = 6.2832 ft., cir. of circle 
6.2832 X 60 = 376.992 ft., distance force moves in 1 min. 
376.992 X 10 = 3769.92 ft.-lb. per min. 
3769.92 



33000 



= .114, horsepower required. 



Problems 



1. The pulley on a washing-machine is 10" in diameter and makes 
100 R. P. M. The belt has an effective pull of 25 lb. What horse- 
power is required ? 

2. The pulley on a kitchen power-table is 6" in diameter and makes 
600 R. P. M. ; the effective pull on the belt is 10 lb. What horsepower 
is required ? 

3. If a motor of 80 per cent efficiency runs the pulley in Prob. 1, 
how many watts does it require? (746 watts = 1 horsepower.) 

4. If a motor of 85 per cent efficiency runs the pulley in Prob. 2, 
how many watts does it require? 

5. When you turn an ice-cream freezer handle 1 ft. long, 50 R. P. M., 
and it requires a force of 8 lb., what horsepower are you producing? 






CHAPTER XXIII 
DYNAMICS 

292. Motion. — Motion is a change of position with refer- 
ence to some other object. 

If you were to look at a book lying near the center of a 
table and were then to close your eyes, and if, while they 
were closed, some one were to change the book to the edge 
of the table, could you tell that it had been moved, when 
you opened your eyes ? You say " Yes " ; for it has changed 
its position with reference to the table. 

Now, if you were to try the experiment again, and the 
person changed the table and let the book remain in the 
center of the table, could you tell whether the book had been 
moved? Some would say " Yes," and some "No." Both 
are right and both are wrong, depending on what is taken as 
a point of reference. Explain. 

293. Newton's Three Laws of Motion. — It always takes 
force to produce, or to change, motion. A chair cannot be 
moved unless some force is applied. Also, anything in mo- 
tion requires a force to stop it or make it change its direction. 

Newton learned this fact and put it into three laws : 

1. Every body continues in a state of rest, or of uniform 
motion in a straight line, unless acted upon by some external 
force. 

2. Every motion is proportional to the acting force, and 
takes place in the direction in which the force acts. 

248 



APPLICATION OF NEWTON'S LAWS 249 

3. To every force there is an equal force in the opposite direc- 
tion. 

294. Meaning and Application of Newton's Laws. — The 
first laic means that if a body is at rest, it has a tendency to 
remain at rest. This is shown when you undertake to move 
a table or some other heavy object, even though it be on cas- 
ters. On the other hand, a body in motion tends to keep on 
going in a straight line. This is illustrated by the skidding 
of an automobile, either around corners or when the brakes 
are set quickly. 

The tendency which a body has to remain at rest, when at 
rest, or to continue in motion, when in motion, is called 
inertia. It is the inertia of your body which throws you 
over in a street car when it turns a corner, or which jerks 
you backward or forward when the car starts or stops 
suddenly. 

The second law means that the resulting motion is doubled 
if the force is doubled, or multiplied by 3 if the force is multi- 
plied by 3, etc. It also means that the object tends to move 
in the direction in which the force acts. 

To illustrate : If you throw a ball with a certain force, 
it will have a certain quantity of motion ; but, if it is thrown 
with twice the force, it will go twice as fast ; also it will go in 
the direction in which it is thrown, if no other force acts 
upon it. 

The third law means that there is always a force, called the 
reaction, which acts in the opposite direction to any given 
force. 

To illustrate this, consider your own weight. This force is 
downward, but the floor pushes upward with the same force ; 
otherwise you would go through the floor. You cannot take 
hold of your shoe-tops and lift yourself, for every pound that 



250 



DYNAMICS 




Figure 277. — A Clothes-line Post 
with Balanced Forces. 



you lift is counteracted by a pound in excess of your weight 
which is pushed downward by your feet. 

295. The Parallelogram of 

Forces. — When two forces act 

upon a body, the body cannot 

move in both directions, but 

moves in the direction of the 

resultant of those two forces. 

For example, a clothes-line 

post, as in Figure 277, cannot 

move in both the directions 

AB and AC, but tends to move along the resultant AR, 

which is somewhere between AB and AC. 

To find the resultant of two forces such as those men- 
tioned above we use what is called the parallelogram of forces. 
First, lay off to scale lines representing the forces in both 
amount and direction. 
(Figure 279.) 

For example, if the force 
AB were 50 pounds, and the 
force AC were 30 pounds, let 
5 inches represent the 50 
pounds and 3 inches repre- 
sent the 30 pounds. Upon 
these two sides construct a 
parallelogram. The diagonal, 
which is 5.83 inches, repre- 
sents the resultant of 5.83 X 
10 = 58.3 pounds. 

In this way the result- 
ant of any two forces 

may be found. If the RouP( ,_ _ ALAB0RAr01? , ExpFRI iN1 

original forces are laid Showing Balanced Forces. 




APPLICATIONS OF PARALLELOGRAM OF FORCES 251 

off to a certain scale, then the length of every line in the 
figure represents the amount of force in that line. 

296. Applications of Parallelogram of Forces. — The 
parallelogram of forces can be used to determine the tension 
in the wires in picture-hanging. 

C 



/ 


' y^ 




,4^ 


© 
CO 


d^ 


J 


4 50*= 5* B 




Scale 1*=10* 


Fig 


ure 279. — The Parallelogram 




of Forces. 





A ) 
/ ^ 


X 


B 










D 20" 



Figure 280. — The 
Parallelogram of 
Forces Applied to 
Picture Hanging. 



Figure 280 shows a picture hanging from a hook in one of the usual 
ways. The distance between the supporting screws in the picture is 
20 in. The distance from the hook to the line of screws is 25 in. Find 
the tension in each wire, if the picture weighs 10 pounds. 
Method: 

If the picture were supported from two hooks (A and B), the wires 
would each be 25 in. long and would support -V- = 5 pounds. 

Since each line in the figure represents the amount of force in that 
line, then 

25 in. = 5 lb. 
1 in. = 2V of 5= A lb. 
The actual wire CD = ^JAC) 2 + {AD) 2 = 

VlO 2 + 25 2 = Vioo + 625 = V725 = 26.9 + 



'. the tension in CD = 26.9 Xl = 5.38 lb. 



Problems 

1. Find the tension in the wire of a picture hung from a hook which 
is 12 in. above the line of the screws in the picture, if the two screws 
are 18 in. apart and the picture weighs 8 lb. 



252 DYNAMICS 

2. What is the tension in a guy-wire for a clothes-line post, if the 
post is 6 ft. high and the guy-wire is set 4. ft. from the base of the post, 
the clothes-line having a tension of 75 lb. ? 

297. Velocity and Acceleration. — Any body in motion 
has a definite speed or velocity — two terms meaning the same 
thing. 

Velocity is the time rate of motion. This means that the 
number of units of distance passed over per unit of time is 
velocity. 

To say that the velocity of a train is 30 miles per hour 

m% \ 
(sometimes written 30 — - ) means it would travel 30 miles 
hr. J 

in one hour, if it ran at that rate of speed. Other units of 

velocity are 

ft. cm. km. , 

j j } exc. 

sec. sec. hr. 

If the speed of an object is the same continuously, it is 
said to have uniform velocity. But if the velocity changes 
it is said to be accelerated. 

Acceleration is the change in velocity per unit time. For 

example, if a body starts from rest and is going at the rate 

ft. ft. 

of 5 -^ at the end of the first second ; 10 — L at the end of 
sec. sec. 

ft. 
the second second; 15 — at the end of the third second, 
sec. 

etc., the motion is said to have an acceleration of 5 ft. per sec- 

ft. 

ond, per second, meaning that it has gained 5 of velocity 

sec. 

every second. 

Acceleration is either positive or negative, according as the 

change in velocity is an increase or a decrease. 



UNIFORMLY ACCELERATED MOTION 253 

The pull of gravity gives all bodies an acceleration down- 
ward of 32.2 ft. per second, per second, or 980 cm. per second, 
per second. This is called the acceleration due to gravity, 
and is represented by the letter g. 

298. Uniformly Accelerated Motion. — When a body is 
uniformly accelerated, it is very often desirable to find : 

(1) The velocity (v) in terms of the acceleration (a) and 
the time (/) which the body has traveled — 

v = at; 

(2) The distance (S) which the body has traveled in terms 
of the acceleration (a) and the time (t) which the body has 
traveled — 

S = i at 2 ; 

(3) The distance (d) which the body has traveled in any 
particular second in terms of the acceleration (a) and the 
second (/) in question — 

d = | a (2 t - 1) ; 

(4) The velocity (v) in terms of the acceleration (a) and the 
distance passed over (S) — 

v 2 = 2 aS. 

The following problems illustrate the use of these formulae : 

Problem (1) : What is the velocity of an automobile at the end of 
5 seconds, if it has an acceleration of 2 ft. per second, per second ? 

Method : 

v = at 

ft 
: . v = 2-5 = 10-^ (ans.) 
sec. 

Problem (2) : How far will a train travel in 10 seconds, if it has an 
acceleration of ^ ft. per second, per second ? 
Method : 

S =\a? 
.-. S = i . i. 1Q2 = i . i . 100 = 25 ft. (ans.) 



254 DYNAMICS 

Problem (3) : How far will a train travel during the 8th sscond after 
starting, if it has an acceleration of ^ ft. per second, per second ? 
Method : 

d = \a (2 t - 1) 



_ i i 

— 2 ' 2 



(2-8-1) =J. |(16-1) 



= £.£.15 = 3f/*. (ans.) 

Problem (4) : What is the velocity of an automobile after it has gone 
25 ft., if it has an acceleration of 2 ft. per second, per second? 
Method : 

v* = 2 aS 
.*. v 2 = 2.2-25 = 100 

„ = VlOO = 10 — (ans.) 

sec. v ' 

All the examples above were given in the English system. 
The same formulae and methods of solution are used in the 
metric system. Instead of feet use centimeters. 

Since the pull of the earth gives all bodies a uniform accel- 
eration, these same formulae apply to freely falling bodies. 

For falling bodies the above formulae may be written and 
used in the special forms : 

v = gt. 

S = \ gi\ 

d = ig(2t- 1). 

v 2 = 2 gS. 

299. Momentum. — The quantity of motion which a body 
possesses is called momentum. It is measured by multiplying 
the mass of a body by its velocity. Thus an automobile 

weighing 2500 lb. and going 20 -y- 1 - has 2500 X 20 = 50,000 

hr. 

lb. -miles per hour of momentum. 

Likewise, a baseball weighing 5 oz. and going 100 ft. per 
sec. has a momentum of yg • 100 = 31 J lb. -ft. per sec. 

There is no definite unit for momentum, so terms such as 



FORCE TO OVERCOME INERTIA 255 

Ib.-mi. per hr., lb. -ft. per sec, etc., have to be used. In com- 
paring momenta, care must be taken that they are ex- 
pressed in the same units. 

300. Force to Overcome Inertia. — By Newton's first law 
of motion every body tends to remain at rest or to continue in 
a straight line at a uniform speed unless some force acts 
upon it ; hence a force setting a body in motion (or stopping 
its motion) must overcome this inertia, together with the 
other forces acting upon the body, such as friction, weight, 
etc. 

The force to overcome inertia is proportional to both the 
mass of the body and the acceleration given it. Thus : 

F = Ma (1) 

or b = (2) 

9 

If the mass is given in grams and the acceleration in centi- 
meters per second, per second, equation (1) gives the force in 
dynes. If the weight is given in pounds or grams and the accel- 
eration in feet per second, per second, or centimeters per second, 
per second, equation (2) gives the force in pounds or grams re- 
spectively. 

Thus a girl weighing 110 lb. and standing in an elevator going down 
with an acceleration of 2 ft. per second, per second, will apparently 
weigh 103.2 b. 

F= Wa 

F = 110i2 =6 . 8 lb. 

32.2 

.'. she weighs 6.8 lb. less than 110 = 103.2 lb., her apparent weight. 

If the elevator were going up with an acceleration of 2 ft. per sec- 
ond, per second, she would weigh 6.8 lb. more, or 110 + 6.8 = 116.8 lb., 
her apparent weight. 



256 



DYNAMICS 



The force required to overcome the inertia of any body can 
be found in a similar manner. 

301. Force to Overcome Friction. — Excepting the mo- 
tions of the heavenly bodies, all motions are opposed 
by a certain amount of friction, so that the force 
changing the motion of a body must overcome the fric- 
tion besides overcoming inertia and other forces, such as 
weight, etc. 

In calculating the force necessary to produce motion of a 
body, each part must be calculated separately and the results 
added. 

302. Centrifugal Force. — Any body moving in the cir- 
cumference of a circle (Figure 281) tends to fly away from 

the center. This is due to Newton's 
first law of motion. Explain. 

The force tending to throw^ the body 
away from the center is called the 
centrifugal force. 

A pail of water may be swung in a 
vertical plane without spilling the 
water on account of the centrifugal 
force. Centrifugal force causes ve- 
hicles to skid around corners. The 
cream separator uses centrifugal force to separate the cream 
from the milk. This can be done because cream is lighter 
than plain milk. 

303. Energy of Motion. — All bodies in motion have 
energy due to that motion. An automobile moving 
60 mi. per hour will do more damage, if it smashes into 
a building, than if it were running 10 mi. per hour. A 
hammer swung with the arm will drive a nail farther than 
if the hammer were just laid on the nail. These are all 




Figure 281. — Centrif- 
ugal Force. 



GRAVITATION 



257 



illustrations of 
Energy (KE). 



energy of motion, usually called Kinetic 



KE 



Wv 2 

2g 



The above formula will give the kinetic energy in foot- 
pounds if IT' is expressed in pounds; v = feet per second; 
and g = 32.2. 

304. Gravitation. — Every bit of matter in the universe 
exerts a pull on every other bit of matter. This pull is called 
gravitation. 

The earth, being a very large bit of matter, exerts a pull 
on all objects on or near it. This pull is called the weight 
of the object. 

Newton formulated three laws, called Newton's three laws 
of gravitation. They are : 

1. The iceight of an object at any given place is directly pro- 
portional to its mass. 

2. The iceight of an object above the surface of the earth is 
inversely proportional to the square of the distance from the 
center of the body to the 

center of the earth. 

3. The weight of a body 
below the surface of the 
earth is directly propor- 
tional to the distance be- 
tween the center of the body 
and the center of the earth. 

The first law needs no explanation. The second law can 
be made more clear by the use of Figure 282. 

It will be seen that the farther the body is away from the 
earth, the fewer are the lines of gravitation which pass 




Figure 282. — Illustrating the Second 
Law of Gravitation. 



258 



DYNAMICS 




through it. This is why the pull gets less as the distance 
gets greater. 

Figure 283 illustrates the third law. A body inside the 
earth has part of the earth (A ) pulling to the right, while the 

other part (B) pulls to the 
left. Thus we see that the 
resulting force becomes 
smaller as the distance be- 
tween the center of the body 
and the center of the earth 
becomes smaller. 

305. Pendulum. — A pen- 
dulum is a body supported 
from a pivot and free to 
swing because of its weight. 
(Figure 284.) L represents 
the length of the pendulum; 
a, the amplitude of the swing; g, the acceleration due to 
gravity ; t, the time of the pendulum — the time it takes the 
pendulum to move from one side of the swing to the other. 
There are four laws governing the time of a pendulum : 

1. The time is independent 
of the mass. 

2. The time is independent 
of the amplitude. 

3. The time is directly pro- 
portional to the square root of 
the length. 

4. The time is inversely proportional to the square root of 
the acceleration due to gravity. 

The pendulum is used to regulate clocks, etc. To make 
a clock run faster, shorten the pendulum. 



Figure 283. — Illustrating the 
Third Law of Gravitation. 




Figure 284. — The Pendulum. 



CHAPTER XXIV 
MECHANICS OF FLUIDS 

306. The Three States of Matter. — All matter exists in 
one or more of three states — solid, liquid, or gas. Some 
substances are found in all three states. Water is the most 
common of these. Other substances existing in the three 
states are iron, copper, lead, mercury, etc. 

The apparent difference between the three states of matter 
is as follows : 

1. A solid has a definite shape and volume. 

2. A liquid has a definite volume, but takes the shape of 
the containing vessel. 

3. A gas has neither a definite shape nor volume, but 
takes the shape of the containing vessel and fills it com- 
pletely. 

The theoretical difference between the three states of 
matter depends upon the molecular construction of the 
substance in these different states. 

In a solid, the molecules are close together and are held 
firmly together by a force called cohesion. This force is 
sufficient to keep the molecules from changing their relative 
positions, but it allows them to vibrate. 

In a liquid, the molecules are farther apart, and the force 
of cohesion is not so great. The molecules can slide over 
one another, but still the force is great enough to keep them 
from separating. 

259 



260 



MECHANICS OF FLUIDS 




Figure 285. — Liquids and 
Solids in Pipes. 



In a gas, the molecules are far apart, the force of cohesion 

is too small to count, and the molecules fly about with perfect 

freedom, bumping against one 
another and the sides of the con- 
taining vessel. 

307. Gases and Liquids through 
Pipes. — The fact that gases and 
liquids have no definite shape 
makes it possible to deliver them 
through pipes. 

Consider the two pipes (a) and 
(b) (Figure 285) filled with water 

and chunks of coal, respectively, and then a force put 

on both of them. In the first case, the water molecules 

would slide over one another at the bend 

of the pipe, and so would flow around the 

bend ; but, in the second case, the chunks 

of coal would not slip past one another, 

but would push against the end of the 

pipe and would clog the pipe. A gas 

would act in the same way as the water. 

Thus we see why it is possible to de- 
liver gas and water through pipes, but 

why we have to haul our coal, wood, and 

all other solids. 
308. Pressure. — Figure 287 shows a 

cylinder with water in it, and a piston 

(K) being forced against the water with 

a force of 100 lb. 

It will be seen that the water will Figure 286. — Pres- 

push on the end of the cylinder with a ^* E p^ RE S ExT1N _ 

force of 100 lb. If the end of the cylinder guisher 




. TO OPERATE 
1WN HANDLE TO UN 



THE HYDRAULIC ELEVATOR 



261 



= 4 lb. (P, Figure 287.) 



w 



. 



2.5 sq. in. 



r— —————— - 



Figure 287. — Meaning of 
the Term "Pressure." 



has an area of 25 sq. in., this 100 lb. will be distributed over 
the total 25 sq. in. Thus each square inch will receive 
100 
25 

The force on the one square 
inch is called the pressure. 

Pressure is the force per unit 
area. It is found by dividing the 
force bv the area of the surface. 

'-! 

Force applies to the total area, while pressure applies 
only to unit area. 

309. Pascal's Law. — In Figure 287 the water would 
press not only on the end of the cylinder, but also on 
the sides ; that is, every square inch of surface would also 

have a force of 4 lb. ; 
or, as w r e say, the pres- 
sure would be 4 lb. per 
square inch. 

Pascal stated these 
facts in the form of a 
law : The pressure on a 
confined liquid is trans- 
mitted undiminished in 
all directions, and acts at 
right angles to all sur- 
faces. 

310. The Hydraulic 
Elevator. — The hydrau- 
lic elevator (Figure 288) 
uses the principle ex- 




Figure 288. — The Hydraulic Elevator. 



262 



MECHANICS OF FLUIDS 



pressed by Pascal's Law. A large piston (P) on the bottom 
of the elevator fits into a cylinder in the ground. A pipe 
(K) runs down the side of the cylinder and enters it at the 
bottom. 

To go up, the stopcock (S) is turned so that water enters 
the pipe (K) from the water-main (a). The water flows 
down the pipe (K) and into the cylinder, pushing up on the 
piston (P). Since the pressure in the water-main is about 
60 lb. per square inch, there is also a pressure of 60 lb. per 
square inch exerted on the bottom of the piston. 

If this piston contains 100 sq. in., the elevator will be 
pushed up with a force of 60 X 100 = 6000 lb. 

To come down, the stopcock is turned so that no more 
water can get into the pipe, but the pipe is opened to the 
outlet or sewer. The weight of the ele- 
vator pushes the water out, and the 
elevator comes down slowly. 

311. Breaking Jugs or Fruit Jars. — 
Jugs and fruit jars are very often broken 
by filling them with a liquid and then 
forcing in the stopper or pressing on the 
lid. The force is applied to a small area, 
and this produces a large pressure. This 
pressure being transmitted to the total 
area of the sides and bottom is sufficient 
to break the jar. 

312. A Liquid in an Open Vessel. — 
When a liquid is in an open vessel, the 
pressure acts in all directions, just as in 
the closed vessel, but the amount of 

pressure depends on the weight of the liquid above, and 
not on an outside force. 




Figure 289. — Pres- 
sure in an Open 
Vessel. 



A LIQUID IX AN OPEN VESSEL 



263 



Figure 289 shows water in a rectangular tank 2 ft. square and 6 ft. 

deep. It is seen that the total weight of the water rests on the bottom. 

Since water weighs 62^ lb. per cubic foot, the force on the bottom is 

2 x 2 X 6 = 24 cu. ft. 
24 X 62| = 1500 lb. 

Since the 1500 lb. is on 4 sq. ft., 



Pressure 



1500 



= 375 lb. per square foot. 



or Pressure = — = 2.61b. per square inch. 
144 F H 

It has been proven that the pressure on the bottom of a 
vessel has nothing to do with the shape of the vessel, but 
depends solely upon the depth of 
the liquid and the area of the 
base. 

Problem : Find the pressure on the 
bottom of the irregular vessel filled 
with water. (Figure 290.) 

Assume a column of water 6 ft. 
high standing on a base one foot 
square. 

Then its 
weight = 1 X 1 X 6 X 62^ = 375 lb. 

Thus the pressure is 375 lb. per 
square foot, regardless of the shape of 
the vessel. 
375 
144 




2.6 lb. per square inch. 



Figure 290.- 
Irregular 
Vessel. 



Pressure in an 
Shaped Open 



Rule : To find the pressure in pounds per square foot of a 
liquid in an open vessel, multiply the height (h) in feet, by the 
weight of the liquid per cubic foot (D). 



P = h-D. 



264 MECHANICS OF FLUIDS 

If the pressure is wanted in pounds per square inch, 
divide by 144. 

£•£ 
144 

Problem : What is the pressure in pounds per square inch 20 ft. 
below the surface of water ? 

p= hD 
144 

D 20 X 62.5 oao . , 

= iH = pounds per square inch. 

Problem : What is the pressure 3 ft. under mercury, if it is 13.6 
times as heavy as water ? 

144 

p 3 X 62.5 X 13.6 17 _ , . , 

F = — — — ■ — = 17.7 pounds per square inch. 



Problems 

1. The water in a tank stands 18 ft. above a faucet. What is the 
pressure at the faucet ? 

2. How high does the water rise in the spout of a teakettle? 

3. Could a large tank of water, on a level with the second story, and 
a hose, be used to fight fire on the third story? Why? 

4. What is the pressure on a deep-sea diver when he goes down 
180 ft., if sea water is 1.1 times as heavy as fresh water? 

5. What is the pressure at a faucet on the third floor, if the pressure 
in the water-main in the basement 45 ft. below is 60 lb. per square inch ? 

313. Air-Pressure. — Air, like water, has weight, but not 
so great as water. The atmosphere is estimated to reach 
from 300 to 400 miles above the surface of the earth ; and 
all this great weight of air above is resting on the lower 
layers, producing a pressure just as the weight of the water 
above produces a pressure on the water beneath. 



THE SIMPLE BAROMETER 



265 



r\ 



At sea-level the air-pressure is normally 14.7 lb. per square 
inch. Places above sea-level have less pressure, because 
there are fewer layers of air resting on them. The upper 
layers are not so heavy, since they are less compressed, 
consequently the pressure falls rapidly 
as you rise above sea-level. 

The air-pressure is measured by an 
instrument called the barometer. 

314. The Simple Barometer. — A 
simple barometer may be constructed in 
this way : Take a glass tube about 32 in. 
long, closed at one end, and fill it with 
mercury. Then invert it in a cup of 
mercury, being careful not to let in any 
air. (Figure 291.) 

The mercury will fall away from the top 
of the tube, and stand at 30 in., more or 
less, according to the air-pressure. The 
space above the mercury in the tube is 
almost a vacuum, since there is nothing 
in it except a little mercury vapor. 

The pressure of the mercury in the tube 
is exactly balanced by the pressure of 
the air on the surface of the mercury in the cup. This 
pressure can be expressed in inches of mercury, centimeters 
of mercury, pounds per square inch, or grams per square 
centimeter. 

If the pressure is wanted in inches of mercury, or centimeters 
of mercury, it is read directly from the column of mercury ; 
but if it is wanted in pounds per square inch, or grams per 
square centimeter, it has to be calculated as one calculates 
the pressure in a liquid. 




Figure 291. — The 
Simple Barometer. 



266 



MECHANICS OF FLUIDS 




Figure 292.— The WEIGHT of the Air Makes it Possible to Fly. 

Example : What is the pressure in pounds per square inch, when the 
barometer reads 28 in. ? 

r, hxD 



144 



28 
12 



ft. 



D = 62.5 X 13.6 = 850 lb. per cubic foot. 

11 X 850 28 X 850 



.'. P = 



144 



12 X 144 



= 13.77 lb. per square inch. 



315. The Commercial Barometer. — The commercial ba- 
rometer, which is used for accurate readings of the air- 
pressure, is a modified form of the simple barometer. 

Figure 293 is a diagram of this instrument. The glass 
tube is inclosed in a brass tube having part of it cut away 
so that the glass tube can be seen at the upper end. The 



THE COMMERCIAL BAROMETER 



267 



G± 



mercury cup has a rubber or leather bottom, so that it can 
be raised or lowered by a set-screw (a) . 

A small movable scale (V), called a vernier, is operated 
by a set-screw (b), and slides at the side of a scale (S) marked 
off in inches and tenths 
of inches. 

To make a reading : 
First, adjust the mer- 
cury in the cup with 
-V the set-screw (a) so that 
the top of the mercury 
just touches the point 
of the ivory plug (P). 
This point is the zero 
of the scale (S) . 

Second, slide the ver- 
nier (V) by means of 
screw (6) so that the 
bottom of the vernier 
is just at the top of 
the mercury in the 
tube. 

Third, read the scale 
(S) and the vernier ( V) . 
a Figure 295 shows an 

Figure 293. — Diagram enlarged drawing of the 

of the Commercial 
Barometer. 

nier (V) 

First, note where the zero of the vernier (V) comes on the 
scale (S). In the figure it is past 28.3, and not quite to 28 A ; 
then the scale reading is the smaller of these, or 28.3. 

Second, note where a mark on the vernier (V) coincides 



scale (S) and the ver- 



Figure 294. — 
Photograph of 
a Barometer. 



268 



MECHANICS OF FLUIDS 



29 



s — 



10 

5 

V 

-o 



with a mark on the scale (S). In the 
figure it is 5 on the vernier. (It 
makes no difference which one on the 
scale.) This determines the next 
figure to be annexed to the scale 
reading, which makes the completed 
reading. Thus the reading in Figure 
295 is 28.3 with 5 annexed, or 28.35". 
316. Weather Maps. — Weather 
conditions are usually accompanied 
by certain air-pressure and tempera- 
ture changes. Knowing this fact, the 
government has a branch of the De- 
partment of Agriculture called the 
United States Weather Bureau, part of whose duties it is 
to make weather maps and from them send out weather 
forecasts. 



28 



Figure 295. — Enlarged 
Drawing of the Ver- 
nier of a Barometer. 




Figure 296. — A Typical Weather Map. 



THE LIFT-PUMP 



269 






The Weather Bureau has stations established all over 
the United States, and every 24 hours these stations report 
to the head office at Washington, D. C, on the weather 
conditions. Some of the things reported are barometer 
reading (reduced to normal conditions), temperature, clear, 
cloudy, rain, or snow, direction and velocity of wind. These 
reports are then summarized and reported back to all the 
stations. Each station then draws up a weather map and 
forecasts the local weather for the next 48 hours. 

A weather map (Figure 296) is made by drawing heavy 
lines, called isobars, through all stations of equal pressure; 
dotted lines, called isotherms, through all stations of equal 
temperature ; an arrow at 
each station, indicating 
the direction of the wind ; 
and small circles marked 
to show whether it is 
clear, partly cloudy, cloudy, 
rain, or snow, respectively. 
The cloudy areas are 
shaded, the low pressure 
areas are marked "LOW," 
and the high pressure areas 
are marked "HIGH." 

For a further study of 
the weather map read 
some good physical geog- 
raphy. 

317. The Lift-Pump. — 
Figure 296 is a diagram of 
the lift-pump, which is an 

application of air-pressure. Figure 297. — The Lift-pump. 




270 



MECHANICS OF FLUIDS 



The piston (P) works air-tight in the cylinder of the 
pump. When the piston is drawn up, the valve (B) closes, 
and a partial vacuum is left behind the piston. The air- 
pressure, acting on the surface of the water (C, C) in the 
well, forces the water up to fill this partial vacuum. 

On the down stroke of the piston, valve (A) closes and 
(B) opens. After several strokes, the water reaches up 
into the pump. The operation is continued, and the water 
flows through the valves, instead of air. When the water 
gets high enough, it runs out of the spout. 

Sometimes the pump will not start, but has to be 
" primed." This is because the valves or piston will not 
hold air, so water has to be put in to make them air-tight. 

This kind of pump can be used only to pump water from 
shallow wells and cisterns, since the air-pressure will raise 

water only 34 ft. 
under ideal condi- 
tions ; and only 
about 28 ft., practi- 
cally. 

318. The Force- 
Pump. — The force- 
pumps used to drive 
water into mains, 
pressure tanks, and 
fire hose are much 
like the lift-pump, 
only instead of allow- 
ing the water to flow out of the spout of its own accord, it 
is confined in the top of the pump and forced out. (Figure 
298.) 

An air-chamber (C) is attached to the pump, so that the 




Figure 298. — The Force-pump. 



OTHER APPLICATIONS OF AIR-PRESSURE 271 



air, when compressed, acts as a spring to keep the pump 
from bursting and to keep the water flowing between strokes. 

319. The Siphon. — Figure 299 represents a siphon, 
which consists of a tube with its ends in water, at different 
levels. If the tube is completely filled with liquid, the 
liquid will run through the tube from the higher level to 
the lower. 

The air-pressure on the surface of the water (c) tends to 
lift the water 34 ft. in the tube. Also the same air-pressure 

at (d) tends to lift the ^ ^ 

water 34 ft. on the 
other side of the tube. 
But the water presses 
downward on the two 
sides with a pressure 
of a ft. and b ft., re- 
spectively. This 
leaves a pressure of 
34 - a and 34 - b, 
respectively. Since b 
is greater than a, the 
greater pressure is to- 
wards (6), and the water runs in that direction. The 
greater the difference in (a) and (6), the faster the liquid 
will flow. 

The siphon is used for getting acids out of carboys, cider 
out of barrels, water out of tanks, etc. 

320. Other Applications of Air-Pressure. — Drawing soda 
water through a straw could not be done if it were not for 
air-pressure. The air is drawn out of the straw, leaving a 
partial vacuum, and the air-pressure forces the soda water 
up to take the place of the air. 




W^% 



Figure 299. — The Siphon. 



272 MECHANICS OF FLUIDS 

Ordinary breathing depends upon air-pressure. The 
muscles of the chest act and make the cavity in which the 
lungs are located larger. This reduces the pressure in the 
lungs, and the air is forced in to equalize the pressure. 

Fruit-jar lids are often hard to get off on account of the 
pressure of the air. When the jar is sealed, the liquid 
and air in the jar are hot. On cooling, they both contract, 
thus reducing the pressure inside the jar. The outside 
air-pressure then holds the lid on very tight. Corks 
drawn into bottles in the same way are often hard to get 
out. 

Air-pressure enables the house-fly to stick to the ceiling. 
His feet have tiny pads on them, and when he sets them 
down all the air is squeezed out from under them, and then 
the pressure of the air makes them stick to the wall or ceil- 
ing. A fly will fall off the side of a bell jar and will crawl 
around on the bottom, if he is put inside and the air is 
pumped out. 

" Suction soles " on gymnasium shoes are similar to the 
foot-pads of the fly. The soles have holes, or depressions, 
on the bottoms, and when the weight of the wearer comes 
down on them, the air is squeezed out, and then the air- 
pressure outside tends to make them " stick." " Suction 
tread " tires work on exactly the same principle. 

321. Boyle's Law. — All gases can be compressed by 
putting pressure on them. That is, more and more gas may 
be forced into the same space, or a certain amount of gas 
may be forced into a smaller space. In either case the 
pressure in the gas is increased. 

On the other hand, a gas will expand if allowed space to 
do it in. In this case the pressure is decreased. 

Boyle stated these facts in a law, called Boyle's Law. 



SURFACE TENSION 273 

The volume of a gas at a constant temperature varies in- 
versely as the pressure exerted upon it. 

This means that if the pressure is doubled, the volume is 
halved; or if the pressure is halved, the volume is doubled, 
etc. 

The law applies to natural or artificial gas used as a fuel. 
The higher the pressure, the more gas there is squeezed into 
a cubic foot ; and, since gas is usually sold by the cubic foot, 
the pressure affects the cost of the gas. 

This change in cost due to change in pressure is not as 
great as some people think. An illustration will show how 
much the effect is. 

Suppose the normal pressure is 6 oz. per square inch. 
(This is the average pressure maintained for natural gas.) 
This means 6 oz. per square inch above atmospheric pres- 
sure. Since atmospheric pressure is about 14.5 lb., or 232 
oz., per square inch, this makes the actual pressure in the 
gas main 232 + 6 = 238 oz. per square inch. 

Xow, if the gas pressure should fall 50 per cent, or to 3 oz. 
above atmospheric pressure, the actual pressure in the main 
would be 238 — 3 = 235 oz. per square inch. 

Thus there will be — as much gas in a cubic foot as 
238 * 

there was at the normal pressure of 6 oz. per square inch. 

The inflation of tires with air under pressure is also an 
application of Boyle's Law. 

322. Surface Tension. — All liquids act as if they have a 
" skin " or " membrane " stretched over their surfaces. A 
needle may be laid on the surface of water (Figure 300), if 
care is taken. The surface of the water is curved under the 
needle just as if there were a cover over the water. This 
apparent " skin " or membrane is called surface tension. 



274 



MECHANICS OF FLUIDS 



The fact is, there is no membrane on the liquid. The 
molecules at the surface are exactly the same as inside the 

liquid. Surface tension is ex- 
plained as follows : 

Consider a molecule of water 





Figure 300. — A Needle 
Lying on Water, 



Figure 301. — Surface 
Tension Explained. 



(m, Figure 301) at the surface of the water, 
in quadrants (a) and (d) 
attracts the molecule (???) 
and tends to pull it down- 
ward. As there is no 
water in (b) and (c), — 
but only air, which 
attracts the molecule (???) 
but slightly, — the result- 
ing effect is for the mole- 
cule (???) to be pulled 
toward the center of the 
water, and every other 
molecule on the surface 
is pulled toward the 
center in the same wav. 



The water 




Figure 302. 



Water in Contact with 
Glass. 



CAPILLARITY 



275 



This gives the effect of a stretched covering over the surface 
of the liquid. 

323. Capillarity. — Capillarity is an application of sur- 
face tension. Figure 302 shows water in contact with glass. 
The water against the glass 
is curved up ; because 
glass has a greater attrac- 
tion for water than water 
has for water; therefore 
the glass in quadrant (c) 
pulls the molecule of water 
(???) more than the water 
in quadrant (a). Also the 
glass in (b) pulls (???) more 
than does the air in (d). 
This makes the surface of 
the water curve as shown 
in the figure. 

Figure 303 shows mer- 
cury in contact with glass, 
is curved down. 




Figure 303. — Mercury in Contact 
with Glass. 



rC> 



The mercury against the glass 
Mercury attracts mercury more than 
glass attracts mercury, therefore the mercury in quadrant 
(a) pulls the molecule of mercury (???) 
more than the glass in quadrant (c). 
Also the glass in (6) attracts (???) more 
than the air in (d). Thus the sur- 
face curves downward as shown in the 
figure. 

When a tube is put into a vessel of 
water, the water creeps up the tube, as 
shown in Figure 304. When a tube is 
put into a vessel of mercury, the mercury 




Figure 304. — How 
Water Creeps up 
a Glass Tube. 



276 



MECHANICS OF FLUIDS 



creeps down the tube. (Figure 305.) This is called 

capillarity. 

The steps in this process are as follows : 

When the tube is placed 
in the water (Figure 304), 
the surface of the water 
curves up the glass; but 
since the surface tension on 
the water acts like a rubber, 
covering, the surface straight-, 
ens out; and then curves 
again. This alternation is 
kept up until the weight of 
water in the tube is so great 
that the surface tension is 
not able to lift it and 
straighten out the surface. 

In the case of mercury and 
glass the mercury is pressed 
down (Figure 305), the pro- 
cess being the same as for 
water, except that the sur- 
face curves in the opposite 
direction. 

324. Other Applications of 
Surface Tension. — Rain- 
drops become spherical on 

account of surface tension. The elastic surface tends to pull 

all molecules towards the center, thus producing a sphere. 
Drops of water on a greased surface become spherical for 

the same reason. Similarly, drops of mercury on a table or 

your hand become spherical. 




Figure 305. — How Mercury Creeps 
down a Glass Tube. 



ARCHIMEDES' PRINCIPLE 



277 



Soap-bubbles are thin films of soapy water with a double 
surface tension — one on the inside, and one on the outside. 
Sometimes you can see the water run down between the 
two surfaces. 

The fact that the white of an egg has a high surface tension 
makes it possible to " beat " it into a white fluffy mass. 
This fluffy mass is made up of thousands of tiny bubbles 
which depend on surface tension for their existence. 

Oil is sometimes poured on stormy 
seas to stop the breaking of the 
waves and thus save the ship. The 
three surface tensions act as a 
blanket over the water. Explain 
why there are three surface tensions. 

325. Archimedes' Principle. — 
Archimedes formulated the follow- 
ing principle : 

A body immersed in a fluid loses in 
weight an amount equal to the weight 
of the fluid displaced. 

This principle can be demon- 
strated as follows : Suppose a cube 
1 ft. on an edge be immersed in water so that the top of 
the cube is 5 ft. below the surface. (Figure 306.) Then 
the bottom of the cube is 6 ft. below the surface. The force 
downward on the top of the cube equals 

F = h ■ D A 

F = 5 X 62± X 1 = 312| lb- 

The force upward on the bottom of the cube equals 

F = h • D ■ A 

F = 6 X 62§ X 1 = 375 lb. 




Figure 306. — Archimedes' 
Principle Verified. 



278 MECHANICS OF FLUIDS 

This leaves a force upward of 375 - 312i lb. = 62i lb. 
But 62J lb. is the weight of a cubic foot of water, which is 
also the volume of the cube. 

The illustration above assumed that the body was com- 
pletely submerged. If the weight of the body is less than 
the weight of an equal volume of liquid, then the body will 
sink to a depth where it displaces a weight of liquid equal 
to the weight of the body. 

For example, if a body of one cubic foot weighs 40 lb., it 

40 
will sink in water until it displaces 40 lb. of water, or 

F 62.5 

cu. ft. 

Thus a body heavier than a liquid sinks, and one lighter 
than a liquid floats. 

326. Applications of Archimedes' Principle. — A stone 
submerged in water is much easier to lift than one out of 
water. 

A person in water weighs very little. This makes swim- 
ming possible. Why does the swimmer keep as much of 
his body under water as possible ? 

An egg will sink in fresh water but will float in salt water. 
Explain. 

Grapefruit and oranges may be tested for juiciness by 
dropping them into water. If they are juicy arid heavy, they 
will float very low in the water, but if dry and light, they will 
float high. 

A ship sinks in water until the weight of the water dis- 
placed equals the weight of the ship and its cargo. That is 
the reason why an empty freighter rides high and a loaded 
one rides low in the water. 

327. Density and Specific Gravity. — The term density 
means the mass per unit volume. A cubic foot of water 



PROBLEMS 279 

contains 62 J lb., and a cubic centimeter contains 1 gram. 
Therefore the density of water is 62| lb. per cubic foot, or 1 
gram per cubic centimeter. 

Specific gravity is the ratio of the mass of a body to the mass 
of an equal volume of water. 

n .,, ., mass of body 

Specific gravity 



mass of equal vol. of mate 



Specific gravity is a comparison of the density of a body 
to the density of water. 

Since the density of water in the metric system is nu- 
merically 1 (1 gram per cubic centimeter), the specific grav- 
ity and the density of a body in that system are numerically 
equal. 

By the use of the table on the next page the weight of any 
certain volume of a substance can be found, or the volume 
of any certain weight can be found. 

Example : What is the weight of 25 cu. ft. of copper? 
From the table : 1 cu. ft. copper = 550.6 lb. 

25 cu. ft. = 25 X 550.6 = 13765 lb. 

Example : What is the volume of 1000 lb. of cast iron? 
From the table : 1 cu. ft. cast iron = 449 lb. 

1^0= 2.23 cu. ft. 

449 

Problems 

1. What is the weight of a cedar chest that is made of 2 cu. ft. of 
lumber ? 

2. If a gold chain weighs 30 grams, how many cubic centimeters of 
gold does it contain ? 

3. Why is cork used in life preservers ? 

4. A gallon contains 231 cu. in., and a cu. ft. contains 1728 cu. in. 
What is the weight of a gallon of water ? 



280 



MECHANICS OF FLUIDS 



TABLE OF DENSITIES AND SPECIFIC GRAVITIES OF SOME 
SUBSTANCES 





Density 




Substance 






Specific Gravity 




Lb. per 

Cu. Ft. 


Gms. per c.c. 




Ash (dry) 


43.7 


.70 


.70 


Ash (green) 


52.8 


.84 


.84 


Acetic Acid 


66.4 


1.062 


1.062 


Alcohol 


50.0 


.80 


.80 


Aluminum 


165.6 


2.65 


2.65 


Beech 


53.2 


.69 to .852 


.69 to .852 


Cedar 


35.0 


.561 


.561 


Cork 


15.0 


.24 


.24 


Copper (cast) .... 


550.6 


8.81 


8.81 


Copper (sheet) .... 


555.0 


8.88 


8.88 


Brass 


527.5 


8.38 to 8.44 


8.38 to 8.44 


Gold 


1218.8 


19.50 


19.50 


Hydrochloric Acid . . . 


75.2 


1.22 


1.22 


Iron (wrought) .... 


480.0 


7.68 


7.68 


Iron (cast) 


449.0 


7.20 


7.20 


Lead 


709.6 


11.30 


11.36 


Maple ....... 


46.0 


.75 


.75 


Mercury 


850.0 


13.6 


13.6 


Milk 


64.5 


1.032 


1.032 


Nitric Acid 


76.3 


1.22 


1.22 


Oak 


53.1 


.85 


.85 


Pine 


28.8 


.46 


.46 


Platinum . . . . . . 


1348.8 


21.5 


21.5 


Sea Water . . . . . 


64.4 


1.03 


1.03 


Silver 


656.3 


10.5 


10.5 


Spruce 


31.2 


.5 


.5 


Steel 


590.0 


7.84 


7.84 


Sulphuric Acid .... 


115.1 


1.84 


1.84 


Tin (cast) 


455.8 


7.29 


7.29 


Walnut 


41.6 


.67 


.67 


Water 


62.5 


1.00 


1.00 


Zinc 


431.3 


6.9 


6.9 



METHODS OF FINDING SPECIFIC GRAVITY 281 

5. If there were 12 cubes of gold, 1 in., 2 in., 3 in., etc., on an edge 
respectively, and you were told you could have whichever one you 
could lift at the first trial, which one would you try? Why? 

6. If a bucket containing water is placed on the platform of a set of 
scales and is found to weigh 40 lb., what weight will the scales show if 
a cast iron cube 3 in. on an edge is supported just under the surface of 
the water by a string, care being taken that the cube does not touch 
the bucket ? 

7. How could you find the cubical contents of an egg ? 

8. From the table determine the order of the heaviest substances 
named. 

328. Methods of Finding Specific Gravity. — (1) If it is 
possible to weigh a body and also to determine its volume, 
the density can be found by dividing the weight by the volume. 
If the body can be weighed and the dimensions taken, then 
the weight divided by the volume gives the density. This 
density divided by the density of water gives its specific 
gravity. 

Example : What is the specific gravity of a piece of metal if it weighs 
40 1b., and is 2" X 4" X 12"? 
Solution : 

2 X 4 X 12 = 96 cu. in. 

o« • 96 1 , + 

96 cu. in. = — — - = — cu. ft. 

172S 18 

40 

— = 40 X 18 = 720 lb. per cubic foot. 

18 

Density of water = 62.5 lb. per cubic foot. 

• 720 11 K 

.. — = 11.5 = sp.gr. 

(2) The hydrometer (Figure 307 ) is an instrument used 
to determine the specific gravity of liquids. It is a tube, 
weighted at the bottom, that has a scale marked on the side. 
The depth to which it sinks gives the specific gravity reading. 



282 



MECHANICS OF FLUIDS 



An hydrometer, made to read the specific gravities of liquids 
lighter than water, has the zero of the scale at the bottom, but 

one for liquids heavier than water 
has the zero at the top. Why? 
(3) Another method for find- 
ing the specific gravity of a 
body, and the one generally 
used if the body is irregular in 
shape, is to weigh the body in 
air, and then in water. The 
difference represents the weight 
of the water displaced. Why? 
Then the weight in air divided 
by the loss in weight equals 
specific gravity. 

Example : What is the specific 
gravity of a body which weighs 19 
grams in air and 12 grams in water? 

Solution : 




Figure 307. —The Hydrom- 
eter. 



19 - 12 = 
displaced. 

19 

7 



7 grams, wt. of water 



2.71 = sp. gr. 



(4) Other cases : (a) If the body is lighter than water, a 
sinker must be used ; but the principle is similar. 

(b) If the object is soluble in water, it can be weighed in a 
liquid in which it is not soluble, but whose specific gravity 
is known. 

(c) If it is a liquid whose specific gravity is to be found, a 
sinker, first weighed in air, then in water, and then in the 
liquid, will give the data necessary for finding the specific 
gravity. 



REVIEW PROBLEMS 283 

Explain, with an example, how to find the specific gravity in (a), 
(b), and (c). 

Problems 

1. What is the density and specific gravity of a piece of butter 
which is 2f" X 2h" X 4" and weighs 1 lb.? 

2. What is the specific gravity of an egg, if it weighs 1 oz. in air and 
.1 oz. in water? 

3. What is the specific gravity of a grapefruit, if the following data 
are taken ? Weight of grapefruit in air, with a sinker attached, but in 
water, equals 1.5 lb.; weight of sinker alone in water equals .3 lb.; 
weight of grapefruit in water with sinker attached and in water equals 
.1 lb. 

4. What is the specific gravity of a crystal of a substance, if it 
weighs .24 gram in air, and .05 gram in a liquid whose specific gravity 
is 1.5? 

5. What is the specific gravity of a liquid, if a sinker weighs 12 grams 
in air, 5 grams in the liquid, and 4 grams in water ? 

Review Problems 

1. Define force, work, mechanical advantage, and efficiency. 

2. Classify and describe levers. 

3. If a force of 15 lb. is exerted on the handles of a nutcracker 6 
inches from the pivot when the nut is placed 1^ inches from the pivot, 
what is the pressure on the nut ? 

4. The crank on an awning lifter is 15 inches long, and the radius 
of the axle on which the rope is wound is 1 inch. What force on the 
crank is necessary to lift the awning if it pulls down on the rope with 
a weight of 50 lb. ? 

5. If a piano weighs 600 lb. and is rolled up a plank 16 ft. long 
into a truck 4 ft. high, what force is necessary, ignoring friction? 

6. How fast will the blades of an egg-beater run, if the handle is 
fastened to a wheel with 50 cogs, which in turn drives a wheel, with 
8 cogs, directly connected to the blades, the handle being turned 96 
R. P. M. ? 

7. What horsepower is exerted when a 120-lb. girl climbs a stairs 
15 ft. high in \ min. ? 

8. Define motion. 



284 MECHANICS OF FLUIDS 

9. What are Newton's three laws of motion? 

10. Explain the use of the parallelogram of force. 

11. How far will a train travel in 10 seconds if it has an accelera- 
tion of ^ ft. per second, per second, and starts from rest ? 

12. How long will it take a stone to fall 100 ft. ? 

13. How far will an automobile coast if it has a velocity of 36 ft. 
per second and slows down at the rate of 2 ft. per second, per second ? 

14. What is the apparent weight of a girl going up in an elevator 
which is increasing its speed at the rate of 3 ft. per second, per second, 
if her actual weight is 110 lb. ? 

15. Give two uses of the pendulum. 

16. Explain why gases and liquids can be delivered through pipes 
while solids cannot. 

17. How does force on a surface differ from pressure on a surface? 

18. What is the pressure in pounds per square inch at the bottom 
of a tank of water 8 ft. deep ? 

19. If the water main pressure is 60 lb. per square inch, how high 
will the water rise in a pipe ? 

20. Why do high buildings have ext a pumping systems of their 
own? 

21. If you were to supply water to a house, from an open tank, where 
would you locate the tank? 

22. Give five applications of air-pressure. 

23. Explain capillarity. 

24. State Archimedes' principle. 

25. What is meant when we say the specific gravity of brass is 8.3 ? 

26. Why will an egg sink in fresh water and float in salt water ? 

27. How could you test a grapefruit for juiceness in a simple manner ? 

28. What is the specific gravity of an egg, if it weighs 1.1 oz. in 
air, and .08 oz. in water ? 



APPENDIX 



I. Freezing and Boiling Points of Some Common Substances 
Under Normal Atmospheric Pressure 



Substance 



Oxygen . . . 
Ammonia . . 
Ether .... 
Methylic Alcohol 
Distilled Water 
Acetic Acid . . 
Turpentine . . 
Fat, Oil, etc. . 
Mercury . . . 



Freezing Point 



Centigrade 

-235° 

- 75° 

- 113° 

- 112° 

0° 

- 17° 

- 27° 

- 33° 

- 38.8° 



Boiling Point 



Centigrade 

- 182° 

- 39° 
35° 
66° 

100° 
117° 
157° 
210° 
357° 



II. Vapor Tension of Water 

Temperatures Given in Centigrade Degrees, and Vapor Tension in 
Centimeters of Mercury 



Temperatures 


Vapor Tensions 


Temperatures 


Vapor Tensions 


- 10 


.22 


3 


.57 


- 9 


.23 


4 


.61 


- 8 


.25 


5 


.65 


- 7 


.27 


6 


.70 


- 6 


.29 


7 


.75 


- 5 


.32 


8 


.80 


- 4 


.34 


9 


.86 


- 3 


.37 


10 


.92 


_ 2 


.39 


11 


.98 


- 1 


.42 


12 


1.05 





.46 


13 


1.12 


1 


.49 


14 


1.19 


2 


.53 


15 


1.27 



285 



286 



HOUSEHOLD PHYSICS 
II. Vapor Tension of Water — Continued 



Temperatures 


Vapor Tensions 


Temperatures 


Vapor Tensions 


16 


1.35 


30 


3.15 


17 


1.44 


31 


3.34 


18 


1.54 


32 


3.54 


19 


1.63 


33 


3.74 


20 


1.74 


34 


3.96 


21 


1.85 


35 


4.18 


22 


1.97 


36 


4.42 


23 


2.09 


37 


4.67 


24 


2.22 


38 


4.93 


25 


2.35 


39 


5.20 


26 


2.51 


40 


5.49 


27 


2.65 


41 


5.79 


28 


2.81 


45 


7.14 


29 


2.98 


100 


76.00 



III. Table of Specific Heats of Some of Our Most Common Substances 
Substance Specific Heat 

Aluminum 22 

Brass 094 

Copper .095 

Iron 1138 

Mercury 038 

Lead 031 

Ice . .5 

Air (at constant pressure) 2375 

Hydrogen (at constant pressure) 3.4 

Steam (at constant pressure) 48 

IV. Table of Coefficients of Linear Expansion 
Substances Coefficient 

Aluminum 0000222 

Brass 0000187 

Copper 000017 

Glass 0000083 

If the range in temperature is given in Fahrenheit degrees, then the 
above coefficients must be multiplied by f . 



Substances 


Coefficient 


Iron . . 


.0000112 


Platinum . 


.0000088 


Steel . . 


.000013 (tempered) 


Steel . . 


.000011 (untempered) 



L 



APPENDIX 
V. Sources of Heat 



287 



Material 


Kind 


Heat Value 




[Hard 

{Soft 


14000 B. T. U.'s per lb. 


Coal 


12000 B. T. U.'s per lb. 




I Coke 


14000 B. T. U.'s per lb. 


Wood 


( Hard 


8400 B. T. U.'s per lb. 




J Soft 


8600 B. T. U.'s per lb. 


Gas 


J Natural 
[ Artificial 


1200 B. T. U.'s per cu. ft. 


600 B. T. U.'s per cu. ft. 




f Kerosene 
{ Naphtha 


20000 B. T. U.'s per lb. 


Oils 


20000 B. T. U.'s per lb. 




[ Crude Oil 


18000 B. T. U.'s per lb 


Electricity . . . 




3411.72 B. T. U.'s per kw. hr. 



(Electricity is given in this table, though it is not a fuel.) 
VI. Heat Value of Foods 



Food (edible portion) 


Approximate Measure op 

100-Great-Calory 

Portion 


Weight in 

Ounces op 

100-Great- 

Calory 

Portion 


Almonds 

Apples 

Apricots, fresh 

Asparagus, cooked .... 
Bacon, smoked (uncooked) . 

Bananas 

Beans, baked, canned . . . 

string, canned .... 

lima, canned 

Beef, corned 

dried, salted and smoked . 

porterhouse steak . . . 

ribs, lean 

ribs, fat 

round, free from visible fat 

rump, lean 

rump, fat 

sirloin steak 


• 

15 average 

2 medium 

2 large 

2 servings 

1 thin slice, small . . . 

1 large 

1 small serving (h cupful) 

5 servings 

1 large side-dish . . . 

4 large slices 

1 small 

1 average serving . . . 

1 generous serving . . . 
1 average serving . . . 


0.5 
6.5 
6.1 
7.5 
0.6 
3.6 
2.8 
17.2 
4.6 
1.2 
2.0 
1.3 
1.9 
0.9 
3.1 
1.7 
0.9 
1.4 



288 HOUSEHOLD PHYSICS 

VI. Heat Value of Foods — Continued 



Food (edible portion) 



Approximate Measure of 

100-Great-Calory 

Portion 



Weight in 

Ounces of 

100-Great- 

Calory 

Portion 



Beets, cooked . . . 
Brazil nuts . . . . 
Bread, graham . . . 

toasted 

white homemade 

average 

whole-wheat . . . 
Buckwheat flour . . 

Butter 

Buttermilk . . . . 

Cabbage 

Calf's foot jelly . . . 
Carrots, fresh . . . 
Cauliflower . . . . 

Celery 

Celery soup, canned . 
Cheese, American pale 

American red .• . . 

Cheddar . . . . 

Cottage 

Neufchatel. . . . 

Roquefort . . . . 

Swiss 

Chicken, broilers . . 

Chocolate 

Cocoa 

Cod, salt 

Corn, green .... 
Corn meal .... 
Crackers, graham . . 

soda 



3 servings 

3 average size 

1 thick slice 

2 medium slices (baker's) . . 

1 medium slice 

1 thick slice 

1 thick slice 

\ cupful 

1 tablespoonful (ordinary pat) 
If cupfuls (If glasses) . . . 

2 servings 



2 medium 



2 servings 

If cubic inches .... 
If cubic inches .... 
If cubic inches .... 

4 cubic inches (f cupful) 
If cubic inches {\ cupful) 
If cubic inches .... 



1 large serving .... 
1 generous half square . 



water . . . . 
Cranberries, cooked 

Cream 

Cucumbers . . . 
Dates, dried . . . 
Doughnuts . . . 



1 side-dish . . 

2 tablespoonfuls 

3 crackers . . 
3 crackers . . 

3 crackers . . 
f cupful . . . 
\ cupful . . . 
2 large . . . 

4 medium . . 
f doughnut . . 



8.9 
0.5 
1.3 
1.2 
1.3 
1.3 
1.4 
1.0 
0.5 
9.9 

11.2 
4.1 
7.8 

11.6 

19.1 
6.6 
0.8 
0.8 
0.8 
3.2 
1.1 
1.0 
0.8 
3.3 
0.6 
1.0 
3.4 
3.6 
1.0 
0.9 
0.9 
0.9 
7.5 
1.8 

20.3 
1.0 
0.8 



APPENDIX 



289 



VI. Heat Value of Foods — -Continued 



Food (edible portion) 



Approximate Measure of 
100-Great-Calory 

PORTION 



w eight ix 

Ounces of 

100-Great- 

Calory 

Portion 



Eggs, uncooked 

Farina 

Figs, dried 

Flour, rye 

wheat, entire 

wheat, graham 

wheat, average high, medium 

Gelatin 

Grapes 

Haddock 

Halibut steaks 

Ham, fresh, lean 

fresh, medium 

smoked, lean 

Herring, whole 

Hominy, uncooked .... 
Lamb, chops, broiled . . . 

leg, roast 

Lard, refined 

Lemons 

Lettuce 

Liver, veal, uncooked . . . 
Macaroni, uncooked .... 

Macaroons 

Mackerel, uncooked .... 

salt 

Marmalade, orange .... 
Milk, condensed, sweetened 

skimmed ....... 

whole 

Molasses, cane 

Muskmelons 

Mutton, leg 

Oatmeal, uncooked .... 

Olives, green 

Onions, fresh 

Oranges 



lh medium or 2 small 



1 large . 
| cupful 



j cupful . . . 
| cupful . . . 
4 tablespoonfuls 
1 large bunch 



1 average serving 
1 average serving 



I cupful 

1 small chop .... 
1 average serving . . 

1 tablespoonful (scant) 
3 medium 

50 large leaves . . . 

2 small servings . . . 
\ cupful (4 sticks) . . 

2 

1 large serving . 



1 tablespoonful . . 
lyV cupfuls . . . 
1\ cupfuls .... 

f cupful (half glass) 

^ cupful .... 

\ large serving . . 

1 average serving . 
| cupful .... 
7 to 10 

2 medium .... 
1 very large . . . 



2.4 

1.0 

1.1 

1.0 

1.0 

1.0 

1.0 

1.0 

3.7 

4.9 

2.9 

1.5 

1.1 

1.3 

2.5 

1.0 

1.0 

1.8 

0.4 

8.0 

20.4 

2.9 

1.0 

0.8 

2.5 

1.2 

1.0 

1.1 

9.6 

5.1 

1.2 

8.9 

1.8 

0.9 

1.2 

7.3 

6.9 



290 



HOUSEHOLD PHYSICS 



VI. Heat Value of Foods — Continued 



Food (edible portion) 



Oysters, canned . . 
Parsnips .... 
Pea soup, canned . 
Peaches, canned 

fresh 

Peanuts .... 
Peas, dried, uncooked 

canned .... 

green .... 
Pies, apple . . . 

custard .... 

lemon .... 



mince ■ . 

squash 

Pineapples, fresh . . . 

canned 

Pork, chops, medium . . 

fat, salt 

Potatoes, white, uncooked 

sweet, uncooked . . . 
Prunes, dried .... 

Raisins 

Rhubarb, uncooked . . 
Rice, uncooked .... 
Salmon, whole .... 

Shad, whole 

Shredded wheat .... 
Spinach, fresh .... 
Succotash, canned . . . 
Sugar 



Tomatoes, fresh . 
canned . . . 
Turkey. . . . 
Turnips . . . 
Veal, cutlet . . 



Approximate Measure of 

1 00-Great-C alo r y 

Portion 



5 oysters 

1 large 

1 large serving . . . 

1 large 

4 medium .... 
10 to 12 (double kernels) 

2 tablespoonfuls . . 
2 servings .... 

generous serving 

piece 

piece 

piece 

| piece 

£ piece 

5 slices 

1 small serving . . . 
1 very small serving . 



1 medium 



3 large 

-§• cupful (packed solid) . . 
3^ cupfuls (scant) .... 

2 tablespoonfuls .... 

1 small serving 

1 average serving .... 
1 biscuit 

3 ordinary servings (cooked) 
1 average serving .... 

3 lumps, 5 teaspoonfuls 

granulated, 6^ teaspoon- 
fuls powdered . . . 

4 average servings .... 
If cupfuls 

1 serving 

2 large servings (2 turnips) 



Weight in 

Ounces of 

100-Great- 

Calory 

Portion 



4.9 
5.4 
3.5 
7.5 
8.5 
0.6 
1.6 
6.3 
3.5 
1.3 
2.0 
1.4 
1.2 
2.0 
8.2 
2.3 
1.1 
0.5 
4.2 
2.9 
1.2 
1.0 

15.3 
1.0 
1.7 
2.2 
1.0 

14.7 
3.6 



0.9 
15.5 
15.6 
1.2 
9.0 
2.3 



APPENDIX 



291 



VI. Heat Value of Foods — Continued 




Food (edible portion) 


Approximate Measure of 

100-Great-Calory 

Portion 


Weight in 

Ounces of 

100-Great- 

Calory 

Portion 


fore quarter 


1 thick slice 


2.3 


hind quarter 




2.3 


Vegetable soup, canned . . 




25.9 


Walnuts, California . . . 




0.5 


Wheat, cracked 


4 nuts 


1.0 


Whitefish 




2.4 


Zwiebach 




0.8 









VII. Tables of Measurements 

English Lineal Measure 
12 inches = 1 foot 
3 feet = 1 yard 



5§ yards 
320 rods 



1 rod 

1 mile 



Lineal Chain Measure 
7.92 inches = 1 link 
100 links = 1 chain 
80 chains = 1 mile 

Rope and Cable Measure 
6 feet = 1 fathom 

120 fathoms = 1 cable's length 

Cloth Measure 
2.25 inches = 1 nail 

4 nails = 1 quarter 

5 quarters = 1 ell 

Metric Lineal Measure 
10 millimeters = 1 centimeter 
10 centimeters = 1 decimeter 
10 decimeters = 1 meter 
10 meters = 1 dekameter 

10 dekameters = 1 hektameter 
10 hektameters = 1 kilometer 
10 kilometers = 1 mvriameter 



292 HOUSEHOLD PHYSICS 

Equivalent values in English and Metric Lineal Measure 

1 inch =2.54 centimeters 

1 foot = 30.48 centimeters 

1 yard = 91.44 centimeters 

1 rod = 502.92 centimeters 

1 mile = 160,934.72 centimeters 

1 centimeter = .394 inch 

English Surface Measure 

144 square inches = 1 square foot 
9 square feet = 1 square yard 
30j square yards = 1 square rod 
160 square rods = 1 acre 
640 acres = 1 square mile 

Architect's Measure 
1 square = 100 square feet 

Metric Surface Measure 

100 square millimeters = 1 square centimeter 
100 square centimeters = 1 square decimeter 
100 square decimeters = 1 square meter 
100 square meters = 1 square dekameter 

100 square dekameters = 1 square hektameter 
100 square hektameters = 1 square kilometer 
100 square kilometers = 1 square myriameter 

Equivalent values in English and Metric Measure 

1 square inch = 6.45 square centimeters 

1 square foot = 929.03 square centimeters 

1 square yard = 8361.29 square centimeters 

1 square rod = 252,929.04 square centimeters 

1 square centimeter = .155 square inch 

English Measure Volume 

1728 cubic inches = 1 cubic foot 
27 cubic feet = 1 cubic yard 

A standard gallon contains 231 cubic inches, and a standard struck 
bushel contains 2150.42 cubic inches. 



APPENDIX 293 

English Liquid Measure 

4 gills = 1 pint 
2 pints = 1 quart 
4 quarts = 1 gallon 

English Dry Measure 

2 pints = 1 quart 
4 quarts = 1 gallon 
2 gallons = 1 peck 
4 pecks = 1 bushel 

English Fluid Measure 

8 drams = 1 ounce 
16 ounces = 1 pint 
2 pints = 1 quart 
4 quarts = 1 gallon 

Metric Measure of Volume 

1000 cubic millimeters = 1 cubic centimeter 

1000 cubic centimeters = 1 cubic decimeter 

1000 cubic decimeters = 1 cubic meter 

1000 cubic meters = 1 cubic dekameter 

1000 cubic dekameters = 1 cubic hektameter 
1000 cubic hektameters = 1 cubic kilometer 

1000 cubic kilometers = 1 cubic myriameter 

Metric Liquid and Dry Measure 

10 milliliters = 1 centiliter 

10 centiliters = 1 deciliter 

10 deciliters = 1 liter 

10 liters = 1 dekaliter 

10 dekaliters = 1 hektaliter 

10 hektaliters = 1 kiloliter 

10 kiloliters = 1 myrialiter 
The liter contains 1 cubic decimeter or 1000 cubic centimeters. 

Equivalent values in English and Metric Volume Measure 

1 cubic centimeter = .061 cubic inch 

1 cubic meter = 1.308 cubic yards 

1 liter = .908 dry quart = 1.057 liquid quarts 



294 



HOUSEHOLD PHYSICS 



English Measures of Weight 

16 ounces = 1 pound 
2000 pounds = 1 ton 

Metric Measures of Weight 

10 milligrams = 1 centigram 

10 centigrams = 1 decigram 

10 decigrams = 1 gram 

10 grams = 1 dekagram 

10 dekagrams = 1 hektogram 

10 hektograms = 1 kilogram 

10 kilograms = 1 myriagram 

Equivalent values in English and Metric Measures of Weight 

453.6 grams = 1 pound 

VIII. Vibrations of Musical Sounds 

Letter C D E F G A 

Frequency 256 288 320 341| 384 

Interval between con- 
secutive tones . . f V° rf t 
Interval between each 



426f 



1 o 
9 



B C 2 
480 512 



16 
T5 



tone and C . 



1 



1 5 
"8 



IX. Candle-Power of a Few Sources of Light 

Carbon Lamp about f- c. p. per watt 

Tungsten Lamp about |- c. p. per watt 

Nitrogen Lamp about 1 c. p. per watt 

Mercury Vapor Lamp . . . . about 1 c. p. per watt 
Arc Light about 1 c. p. per watt 

X. Terms and Abbreviations in Electricity 



Thing to be Measured 


Unit 


Letter 


Pressure 


Volt 

Ampere 

Ohm 
f Watt 
| Kilowatt 
f Watt-hour 
[ Kilowatt-hour 


E 


Current 

Resistance 

Power 


I 

R 

W 


Electrical Energy 


Kw 

W-hr. 
Kw-hr. 



APPENDIX 



295 



XI. Table of Densities and Specific Gravities of Some Substances 



Substance 



Density 



Lbs. Per 
Cu. Ft. 



Ash (dry) . . . 
Ash (green) . . 
Acetic Acid . . 
Alcohol . . . . 
Aluminum . . . 
Beech . . . . 
Cedar . . . . 

Cork 

Copper (cast) . . 
Copper (sheet) 

Brass 

Gold ...... 

Hydrochloric Acid 
Iron (wrought) 
Iron (cast) . . . 

Lead 

Maple . . . . 
Mercurv . . . 

Milk 

Nitric Acid . . . 
Oak .■•.... 

Pine 

Platinum . . . 
Sea Water . . . 
Silver .... 
Spruce .... 

Steel 

Sulphuric Acid 
Tin (cast) . . . 
Walnut .... 
Water .... 
Zinc 



43.7 

52.8 

66.4 

50.0 

165.6 

53.2 

35.0 

15.0 

550.6 

555.0 

527.5 

1218.8 

75.2 

480.0 

449.0 

709.6 

46.0 

850.0 

64.5 

76.3 

53.1 

28.8 

1348.8 

64.4 

656.3 

31.2 

590.0 

115.1 

455.8 

41.6 

62.5 

431.3 



Gms. Per c. c. 



.70 
.84 
1.062 
.80 
2.65 
.69 to .852 
.561 
.24 
8.81 
8.88 
8.38 to 8.44 
19.50 
1.22 
7.68 
7.20 
11.36 
.75 
13,6 
1.032 
1.22 
.85 
.46 
21.5 
1.03 
10.5 
.5 
7.84 
1.84 
7.29 
.67 
1.00 
6.9 



Specific 
Gravity 



.70 
.84 
1.062 
.80 
2.65 
.69 to .852 
.561 
.24 
8.81 
8.88 
8.38 to 8.44 
19.50 
1.22 
7.68 
7.20 
11.36 
.75 
13.6 
1.032 
1.22 
.85 
.46 
21.5 
1.03 
10.5 
.5 
7.84 
1.84 
7.29 
.67 
1.00 
6.9 



INDEX 



X umbers refer to pages. 



Absolute zero 38 

Absorbers 58 

Acceleration 252, 255 

Additive method in color . . 137 
Air necessary for a person . . 56 

Air pressure 264 

Air pressure, other applications 

of 271 

Alcohol used in thermometers . 4 
Alternating current rectified . 222 

Ammeter 186 

Ammonia used in ice plant . 21, 22 

Amperes 174 

Amplitude . . . 70, 71, 79, 258 

Angle of incidence 96 

Angle of reflection 96 

Annunciator 170 

Anode 213 

Arc lamp, automatic . . 171, 179 

candle power of 130 

Archimedes' principle . . . 277 

applications of 278 

Area 227 

Armature of generator . . . 159 

Artificial ice 8 

plant 21, 22 

rinks 22, 23 

Astigmatism 121 

Atmosphere as a refracting sub- 
stance . . 107 

Atmospheric pressure . . . 8, 9 

Atom 212 

Attraction, law of magnetic . . 147 
Axis 100 

Balance wheel of a watch . . 33 

Barometer .... 265, 266, 267 

Batteries 217 

Beats 78 

Ball, door 165 

Binoculars, field 110 

Blue 132 



Boiling point . . . . 3, 4, 5, 7, 8, 9 

Boyle's law 272 

British Thermal Unit (B. T. U.), 

definition of 10 

Brushes of generator .... 159 

Buzzer 165 

Calory, definition 10 

Calory, great, definition of . . 10 

Camera lens 117 

Camera, life-sized picture . . 122 
Camera, pinhole . . . 116, 117 
Candle power .... 127, 128 

Candle power, measurement of 129 

Capillarity 275 

Carbon lamp, candle power of 130 

color of 136 

Cathode 213 

Center of curvature of mirror . 98 

Centigrade thermometer . . 4 

construction of 4 

Centrifugal force 256 

Charles' law 39 

applications of 39 

applied to baking . . . 39, 40 

applied to clay modeling . . 40 

other applications .... 40 

Chemical energy 221 

Choroid coat of eye . . . . 119 

Chromatic scale 87 

Circuit breaker 169 

City system wiring diagram . 209 

Closed pipes, resonance in . . 83 

Clothes 46 

Clouds 25 

Coal, as a fuel 60, 61 

Cochlea 77 

Coefficient of cubical expansion 35 

linear expansion 29 

linear expansion, table of . . 30 

volume expansion .... 35 

Cohesion 11, 16, 259 



297 



298 



INDEX 

Numbers refer to pages. 



Cold body, differs from hot body 2 

definition of 3 

Color 132 

filters 143 

harmony of 140 

how we see 137 

nomenclature 140 

of opaque objects .... 134 
of transparent and translu- 
cent objects 135 

screens 141 

Colored objects, application of 135 

Colors, cause of 132 

elementary 136 

Commutator of generator . . 160 

Concave mirror 98, 100 

Condensation . . . . 70, 71, 84 
Condenser ....... 203 

Conduction 41, 58 

Conductor, electrical . . 153, 155 

Conductors 41 

Convection 41, 47, 58 

Convection currents 48, 50, 51, 52, 53 
Convex mirror .... 100, 101 

Cornea of the eye 119 

Counter-electromotive force . 193 

Crest 69 

Critical angle 107 

Crystalline lens 120 

Current of electricity . . . . 153 

Dark lantern 124 

Daylight lamp 136 

Decorations, selection of, ac- 
cording to color 136 

Degree, unit used on thermom- 
eter 5 

Density 278, 279, 280 

Dew 25 

Diamond Ill 

Diffused light 125 

Discord 86 

Disks, colored 138 

Dispersion 132 

Distillation 19 

fractional 20 

Domestic science, relation of, to 

physics 2 

Dominant 86 

Double boiler 18 



Drafts in chimney 48 

Dress goods, selection of, ac- 
cording to colors .... 136 

Driven pulley 241 

Driver pulley 241 

Dry cell 219 

Dyes 134 

Dynamics 248 



Ear 

drum . . . . 

external . . . 

how we hear . . 

inner . . . . 

middle . 
Edison storage cell 



77 
77 
77 
77 
77 
77 
223 



Efficiency 235 

Electric clock 169 

curling iron ...... 181 

door latch ...... 172 

flat iron 180 

gas lighter 172 

grill ......... 182 

mangle . 183 

percolator 181 

soldering iron . . . . . 181 

stove 181 

toaster 181 

Electrical current 154 

application of heating effect of 176 

chemical relation of . . . 212 

D C pulsating, made steady 162 

heating effect of 173 

induced 202 

magnetic effect of . . . . 163 

magnetic field about a 163 

motion producing effect of . 184 

through a helix 164 

Electrical energy 174 

generator, simple . . . . 155 

generator, simple AC. . . 156 

generator, simple DC. . . 160 

power 174 

Electrical pressure .... 200 

alternating current .... 159 

amount of 154 

curve of, in A C generator . 157 

curve of, in D C generator . 161 

direct current 159 

direction of 154 



INDEX 

Numbers refer to pages. 



299 



generation of 153 

nature of 154 

of a voltaic cell 215 

stepped up 204 

Electrical units 173 

Electricity 153 

analogous to water . 154, 155 

relation to magnetism . . . 153 

static ........ 224 

Electrodes 217 

Electrolyte 213 

Electrolytic cell 212 

chemical action in ... . 212 

copper sulphate 213 

parts of 213 

sulphuric acid 214 

Electro-magnet .... 164, 165 

applications of 165 

in a coil of wire 201 

other applications of . 172 

poles of 164, 165 

Electro-plating 215 

Electro-typing 215 

Energy 91 

definition of 1 

kinetic 257 

of motion 256 

English system compared to 

metric 229, 230 

of measurement 227 

Ether 57 

vibrations in 3 

waves in 91 

Eustachian tube 77 

Evaporization 23 

Expansion 29 

effect on balance wheel of a 

watch 33 

effect on glass ware ... 34 

effect on pendulum of a clock 32 

effect on water pipes ... 37 

of gases 38 

other effects of 34 

peculiar effects on water 35, 36 

tank 52 

Eye 119, 137 

defective 120 



Fahrenheit thermometer 
Fifth 



4, 5 
87 



Fireless cooker 43 

First class lever .... 234 

Flowing of gases and liquids 260 

Fluorescence 91 

Focal length 99, 112 

Focus 100, 111 

principal 99 

Fog 25 

Foods, heating value ... 63, 64 

Foot-pound 231, 232 

Force 230, 233 

arm 233 

centrifugal 256 

moment 233 

parallelogram of 250 

to overcome friction . . . 256 
to overcome inertia . . . 255 

units of 231 

Forced systems of ventilation 55, 56 

Fourth 87 

Freezing, effect on water pipes 37 

point 1, 4, 5, 8, 9 

point, definition of ... . 6 

points, table of 7 

Frequency . . 70, 71, 73, 79, 86 

Friction 256 

Fuels 68 

Fundamental 80 

Fusion, heat of, definition of . 11 

Galvanometer 185 

Gas, artificial 60 

Gases 259 

Gases and liquids through pipes 260 

Gas meter 62 

Gas, natural 60 

Gelatin, extraction of ... . 9 

Gram-centimeter 231 

Gravitation 257 

Newton's three laws of . . 257 

Gravity cell 220 

Green 



132 



Had 25 

Half-step 87 

Half-tone picture printing . . 140 

Harmony 85 

laws of 85 

of color I 40 



300 



INDEX 

Numbers refer to pages. 



Heat, absorption of ... . 2 

and heat measurement . . 1 

capacity 26 

changed from one form to 

another 2 

definition of 2 

insensible 57 

kinds of 2 

molecular 2 

nature of 1 

necessary for one person . . 64 

of fusion 11 

of fusion, effect on climate 14, 15 
of fusion, protection by . . 14 
of vaporization . . . . 15, 16 
of vaporization, effect on cli- 
mate 21 

of vaporization, other effects 

of 21 

quantity of 10 



radiant . . . 
sensible 
sources of . . 
transference of 
travels . 
units 

units compared 
value of foods 



2 
. 57 
. 60 
. 41 

2 

. 10 

10 

63, 64 



value of fuels . . . . 60, 61, 62 
Helmholtz resonators . . 80, 81 

Horse-power 245 

Hot air heating 50 

Hot bodies, definition of . . . 3 
Hot body, how different from a 

cold body 2 

Hot water bottle 28 

Hot water heating system 51,52,53 

Hot water tank 50, 51 

House circuit, wiring diagram of 209 
Hydraulic elevator .... 261 

Hygrometer 24 

Hypermetropia 120 

Ice cream freezer 13 

Ice cream, making of ... . 6 

Iceless refrigerators .... 22 

Ices, freezer of 13 

Ices, making of 6 

Illumination 127 

problems of 130 



Image 97, 

how to find in a plane mirror 

in a concave mirror 

in a convex mirror .... 

real 

through a converging lens 

113, 114, 

through a diverging lens . . 

virtual 

Incandescent lamp, carbon . . 

gas filled 

mercury vapor 

tungsten 

Incidence, angle of .... 

Incident ray 96, 

Inclined plane . 233, 239, 240, 

height of 

length of 

Index of refraction, absolute . 

relative 

Indigo 

Induction 

coil 

coil, uses of 

mutual 

self 

Inertia 202, 249, 

Insensible heat 

Insulation 

Insulators . . . ' . . . . 
Insulators, electrical .... 
Intensity of illumination . . . 

Intensity of sound 

Interference 

Interval, musical 

Ion 

Ionization 

Iris of the eye 

Isobars ........ 

Isotherms 



112 
97 
99 
100 
98 
112, 
115 
116 
98 
176 
178 
178 
177 
105 
104 
241 
240 
240 
106 
106 
132 
200 
203 
204 
202 
202 
255 
2 

44 
41 

155 

127 



78 
87 
212 
212 
119 
269 
269 



Kilowatt-hours 174 

Kilowatts 174 

Kinetic energy 257 

Kitchen range 49 

Lantern slide 123 

Lead of a screw 244 

Length ........ 227 



INDEX 

Numbers refer to pages. 



301 



Lens, achromatic 133 

condensing 123 

converging Ill 

diverging Ill 

Lenses Ill 

Lever . . . 233, 234, 235, 236, 237 

classes of 234 

Light 91 

velocity of , . . . .92, 93, 94 

Lighthouse reflector .... 109 

Light, nature of 91 

theory of production of . . 91 

waves, propagation of 92 

Line drop 207 

Lines of force 148, 163 

properties of 149 

Liquids . 259 

Lodestone 146 

Long-sightedness 120 

Loudness 79 

Luminous bodies 91 

Machines 233 

Magnet, electro 164 

field of 147 

permanent 152 

poles of 146 

temporary 152 

Magnetic fields, characteristic 152 

needle 163 

poles of the earth .... 147 

substances 150 

Magnetism 146 

of earth 147 

theory of 149 

Magnetized piece of iron com- 
pared to one not magnetized 150 

Magnetizing iron 151 

Magnifying glass 124 

Major scale 86 

triad 86 

Mass 227, 255 

Matter, composition of . . . 2 

definition of 1 

Mechanical advantage . . . 234 

Mechanics of fluids .... 259 

of solids - 227 

Melting point 1 

Mercury in contact with glass 275 

in thermometers . . . . 4, 8 



vapor lamp, candle power of 

vapor lamp, color of 

Meters for A C 

Metric system of measurement 

Miller, Dr. Dayton, of Case 

School of Applied Science 

Mirror 

concave 

convex 

parabolical 

peculiarly shaped .... 

spherical 

Mist 

Mixing colored lights . . . 
Molecular construction of mat- 
ter 

heat 

Molecules \ 

Moment 

Moments, law of 

Momentum 

Motion . . . 230, 231, 248, 

energy of 

formula? for uniformly ac- 
celerated 

Newton's three laws of 248, 

picture machine 

Motor and generator compared 
Motor, compound 

DC 

series 

shunt 

small 

iise of, in home . . 
Music, basis for . . 
Musical instruments . 

interval .... 
Myopia 



194, 



130 
136 
190 
229 

81 
96 
98 

100 
98 

102 
98 
25 

137 

2 
2 
, 11 
233 
234 
254 
252 
250 

253 
249 
123 
191 
194 
190 
194 
195 
197 
199 
85 



87 
120 



Natural system of ventilation 55 

Negative plate 119 

Newton's three laws of gravita- 
tion 257 

■ motion 248, 249 

Nitrogen lamp, candle-power of 130 

Noise 85 

Non-conducting materials . 41, 42 
magnetic substances . . . 150 



Octave 
Ohm 



87 
174 



302 



INDEX 

Numbers refer to pages. 



Ohm's law 175 

Opaque objects . . . .58, 94, 133 

Open pipes, resonance in . . 84 

Optical center 112 

Orange 132 

Overtones 80 

Paints 135 

Parabolical mirror 98 

Parallelogram of forces . . . 250 

Pascal's law 261 

Pencil of rays . . . , . . 96, 97 

Pendulum 258 

laws of 258 

of a clock, compensating . . 32 

Penumbra 95 

Period 70, 71 

natural free . . ... . 76 

Phosphorescence 91 

Photograph, how made . . 118, 119 

Photometer 129 

Physics, definition of ... . 1 
relation of, to domestic 

science 2 

Pigments ....... 135 

colored 138 

mixing colored 138 

Pitch 79 

international standard . . 89 

of screw 244 

standard 88 

Pivot 233 

Plane mirror ..... 97, 100 

Plaster 45 

lath 45 

Polarization . 218 

Power 245 

delivered by pulleys . . . 246 

Pressure, application of water 262 

applied 7 

effect of, on boiling point . 8, 9 

effect of, on freezing point . 8 
how to calculate, in an open 

vessel 262 

kettle 9 

water . 260 

Primary cells 220 

coil 202 

Principal axis 112 

of mirror 98 



Principal focus 99, 112 

Prismatic window glass . . . 109 

Prisms, refracting 109 

Projecting lantern 122 

Pulley .... 233, 241, 242, 243 

Pump, force 270 

lift 269 



Quality of Sound 



79, 80 



Radiant heat 2, 91 

Radiation 41, 57 

Radiators 58 

Radical 212 

Rain 25 

Range, kitchen 49 

Rarefaction 70, 71, 84 

Red 132 

Reflected ray 97 

Reflection .... 96, 103, 104 

Reflection, law of 96 

total . 108 

Reflectors 58 

Refracted ray 104 

Refraction 103 

angle of 105 

index of 105 

law of 104 

Refrigerator 44 

tested 13 

uses of 12 

Repulsion, law of, magnetic . 147 

Resistance 155 

what determines amount of . 155 

Resonance . 76 

in closed pipes ..... 83 

in open pipes 84 

principle of 76, 78 

Resonators 80, 81 

Retina, eye 120, 137 

Roemer's method of finding 

velocity of light .... 92 

Salt on ice, effect of . . . 13, 14 

Saturation point 23 

Scale, chromatic 87 

major 86 

tempered 88 

Sclerotic coat of eye . . . . 119 
Screw 233,244 



INDEX 



303 



Numbers refer to pages. 



Second class lever 234 

Secondary cell 221 

Secondary coil 202 

See, how we 120 

Sensible heat 2 

Shades 138 

Shadows 94, 95 

Sheathing 45 

Short-sightedness . . . ". . 120 

Siphon 271 

Slip-rings 159 

Snow 25 

Solids 259 

Sound 74 

characteristics of ... . 79 
effect of temperature on ve- 
locity of 76 

intensity of 79 

interference of 78 

nature of 74 

quality of 79 

reinforcement of ... 78, 83 
things necessary for ... 74 

velocity of 75, 84 

waves, analysis of . 80, 81 

waves, how they travel . . 75 
waves, photographs of . 82, 83 

Space 227 

Specific gravity 278, 279, 280, 281, 282 
method of calculating . .281, 282 

Specific heat 27 

effect on climate .... 28 

Spectrum, solar 132 

Speed 252 

Spherical mirror 98 

Standard candle 129 

pitch 88 

Starting box 192 

need of 193 

Static electricity 224 

Steam cooker 18 

heating 16, 17, 18 

Storage cell 220 

charging of 222 

drv, lead 222 

Edison 223 

lead, wet 221 

uses of 222 

Studding 45 

Subdominant 86 



Subtractive method .... 138 

Sugar, manufacture of . . . 9 

Surface tension 273 

applications of 273, 274, 275, 276 

Telegraph relay 167 

sounder 166 

system 167 

Telephone 210 

switch board, automatic . • . 172 
Temperature at which water is 

densest 36 

and quantity of heat compared 10 

definition of 3 

Tempered scale 88 

Thermometer, alcohol used in 4 

centigrade 4 

changing from centigrade 
reading to Fahrenheit read- 
ing 5, 6 

expansion involved in . . . 4 

Fahrenheit 4 

fixed points of . . " . " . . 4, 5 

kinds of 4 

mercury r in 4 

relation of centigrade and 

Fahrenheit scales ... 5 

uses of 4 

Thermos bottle 44, 58 

Thermostat 30, 31 

Third class lever 234 

Third, major 87 

Three color printing process . 142 

Three phase system .... 208 

Three primary colors .... 136 

Three states of matter . . . 259 

Time 227 

Tints 138 

Tonic 86 

Transformer . . 204, 205, 206, 207 

advantages and uses of . 206, 207 

Translucent object .... 133 

Transparent object .... 133 

Triad, major 86 

Trough 69 

Tungsten lamp, candle-power of 130 

Umbra 95 

Unison 87 

Units of measurement . . . 227 

velocity 252 



304 



INDEX 

Numbers refer to pages. 



Vacuum 44 

pan 9 

Vapor tension 7 

Velocity 252 

Ventilation 54, 55, 56 

Vernier 267 

Vibrating strings, laws of . . 81 
Vibration, complete . . . .70 

Violet 132 

Volt 174 

Voltaic cell 217 

addwater 219 

closed circuit 220 

Daniell 220 

dry 219 

gravity 220 

open circuit 218 

secondary or storage . . . 220 

sulphuric acid . . . . . 217 

wet salammoniac .... 218 

Voltmeter 187 

Volume 227 

expansion 35 

Walls of houses 45 

Water in contact with glass . 274 
Water vapor in the air 23, 24, 25, 26 

Water, when densest .... 36 



Watt 

Watt-hour 

Watt-hour meter 

Wattmeter 

Wave, characteristics of longi- 
tudinal 70 

transverse 

Wave length . . 70, 71, 73, 

longitudinal . . . 69, 71, 72 

motion 

motion, examples of . . . 

origin of 

transverse . . 69, 70, 71 

velocity of ..... 72 

Weather-board 

Weather maps 

Wedge 233, 

Weighing balance . . . 235, 

Weight 

Weight-arm 

moment 

Wheel and axle . . 233, 238, 

Work 173, 

Work-in 

Work-out 

Work, units of 



174 

174 
191 

188 

71 

, 70 
132 
, 75 
67 
67 
68 
, 92 
, 73 
45 
268 
244 
236 
233 
233 
233 
239 
231 
235 
235 
231 



. 



Yellow 132 







